Importing a Slack export using Administration -> Import -> Import New File type Slack fails.
Importing the same file on RC 3.18.3 works.

Server running on Ubuntu 20.04.3 LTS.
Deployed using snap install rocketchat-server --channel=4.x/stable

Server log:
I20211127-16:34:29.655(0) +-----------------------------------------+ I20211127-16:34:29.656(0) | SERVER RUNNING | I20211127-16:34:29.656(0) +-----------------------------------------+ I20211127-16:34:29.656(0) | | I20211127-16:34:29.656(0) | Rocket.Chat Version: 4.0.5 | I20211127-16:34:29.657(0) | NodeJS Version: 12.22.1 - x64 | I20211127-16:34:29.657(0) | MongoDB Version: 3.6.14 | I20211127-16:34:29.657(0) | MongoDB Engine: wiredTiger | I20211127-16:34:29.657(0) | Platform: linux | I20211127-16:34:29.658(0) | Process Port: 3000 | I20211127-16:34:29.658(0) | Site URL: http://localhost | I20211127-16:34:29.658(0) | ReplicaSet OpLog: Enabled | I20211127-16:34:29.658(0) | Commit Hash: 803e9cf | I20211127-16:34:29.658(0) | Commit Branch: HEAD | I20211127-16:34:29.659(0) | | I20211127-16:34:29.659(0) +-----------------------------------------+ I20211127-16:34:29.659(0) +----------------------------------------------------------------------+ I20211127-16:34:29.660(0) | DEPRECATION | I20211127-16:34:29.660(0) +----------------------------------------------------------------------+ I20211127-16:34:29.660(0) | | I20211127-16:34:29.660(0) | YOUR CURRENT MONGODB VERSION (3.6.14) IS DEPRECATED. | I20211127-16:34:29.660(0) | IT WILL NOT BE SUPPORTED ON ROCKET.CHAT VERSION 5.0.0 AND GREATER, | I20211127-16:34:29.661(0) | PLEASE UPGRADE MONGODB TO VERSION 4.2 OR GREATER | I20211127-16:34:29.662(0) | | I20211127-16:34:29.662(0) +----------------------------------------------------------------------+ {"level":50,"time":"2021-11-27T16:34:56.030Z","pid":9324,"hostname":"fairrc","name":"System","msg":"Exception while invoking method loadMissedMessages 'Invalid room [error-invalid-room]'"} {"level":50,"time":"2021-11-27T16:34:56.051Z","pid":9324,"hostname":"fairrc","name":"System","msg":"Exception while invoking method loadMissedMessages 'Invalid user [error-invalid-user]'"} I20211127-16:35:27.781(0) Unloaded the Apps Framework. {"level":50,"time":"2021-11-27T16:36:26.922Z","pid":9324,"hostname":"fairrc","name":"Slack Importer","err":{"type":"TypeError","message":"Cannot read property 'map' of undefined","stack":"TypeError: Cannot read property 'map' of undefined
at SlackImporter._replaceSlackUserIds (app/importer-slack/server/importer.js:549:18)
at SlackImporter.prepareChannelsFile (app/importer-slack/server/importer.js:43:17)
at app/importer-slack/server/importer.js:209:28
at iterateEntries (/snap/rocketchat-server/1492/programs/server/npm/node_modules/adm-zip/zipFile.js:42:4)
at Object.forEach (/snap/rocketchat-server/1492/programs/server/npm/node_modules/adm-zip/zipFile.js:150:5)
at Object.forEach (/snap/rocketchat-server/1492/programs/server/npm/node_modules/adm-zip/adm-zip.js:425:16)
at SlackImporter.prepareUsingLocalFile (app/importer-slack/server/importer.js:206:8)
at MethodInvocation.getImportFileData (app/importer/server/methods/getImportFileData.js:66:37)
at MethodInvocation.methodsMap. (app/lib/server/lib/debug.js:66:34)
at maybeAuditArgumentChecks (packages/ddp-server/livedata_server.js:1771:12)
at packages/ddp-server/livedata_server.js:1689:15
at Meteor.EnvironmentVariable.EVp.withValue (packages/meteor.js:1234:12)
at packages/ddp-server/livedata_server.js:1687:36
at new Promise ()
at Server.applyAsync (packages/ddp-server/livedata_server.js:1686:12)
at Server.apply (packages/ddp-server/livedata_server.js:1625:26)
at Server.call (packages/ddp-server/livedata_server.js:1607:17)
at DDPCommon.MethodInvocation. (app/api/server/v1/import.js:46:20)
at packages/dispatch_run-as-user.js:211:14
at Meteor.EnvironmentVariable.EVp.withValue (packages/meteor.js:1234:12)
at Object.Meteor.runAsUser (packages/dispatch_run-as-user.js:210:33)
at Object.get (app/api/server/v1/import.js:45:10)"},"msg":"Cannot read property 'map' of undefined"} {"level":50,"time":"2021-11-27T16:36:47.862Z","pid":9324,"hostname":"fairrc","name":"Slack Importer","err":{"type":"Error","message":"Meteor.userId can only be invoked in method calls or publications.","stack":"Error: Meteor.userId can only be invoked in method calls or publications.
at AccountsServer.userId (packages/accounts-base/accounts_server.js:117:13)
at Object.Meteor.userId (packages/accounts-base/accounts_common.js:343:32)
at 0 (app/lib/server/functions/setRealName.js:44:23)
at packages/rate-limit/rate-limit.js:49:19
at Array.every ()
at Rule.match (packages/rate-limit/rate-limit.js:44:8)
at packages/rate-limit/rate-limit.js:249:58
at Array.filter ()
at RateLimiter._findAllMatchingRules (packages/rate-limit/rate-limit.js:249:38)
at RateLimiter.increment (packages/rate-limit/rate-limit.js:228:31)
at app/lib/server/lib/RateLimiter.js:18:16
at saveUserIdentity (app/lib/server/functions/saveUserIdentity.js:39:8)
at ImportDataConverter.updateUser (app/importer/server/classes/ImportDataConverter.ts:245:4)
at ImportDataConverter.insertUser (app/importer/server/classes/ImportDataConverter.ts:266:8)
at app/importer/server/classes/ImportDataConverter.ts:324:26
at Array.forEach ()
at ImportDataConverter.convertUsers (app/importer/server/classes/ImportDataConverter.ts:293:9)
at app/importer/server/classes/ImporterBase.js:198:20
at Meteor.EnvironmentVariable.EVp.withValue (packages/meteor.js:1234:12)
at packages/meteor.js:550:25
at runWithEnvironment (packages/meteor.js:1286:24)"},"msg":"Meteor.userId can only be invoked in method calls or publications."} {"level":50,"time":"2021-11-27T16:36:47.924Z","pid":9324,"hostname":"fairrc","name":"Slack Importer","err":{"type":"Error","message":"Meteor.userId can only be invoked in method calls or publications.","stack":"Error: Meteor.userId can only be invoked in method calls or publications.
at AccountsServer.userId (packages/accounts-base/accounts_server.js:117:13)
at Object.Meteor.userId (packages/accounts-base/accounts_common.js:343:32)
at 0 (app/lib/server/functions/setRealName.js:44:23)
at packages/rate-limit/rate-limit.js:49:19
at Array.every ()
at Rule.match (packages/rate-limit/rate-limit.js:44:8)
at packages/rate-limit/rate-limit.js:249:58
at Array.filter ()
at RateLimiter._findAllMatchingRules (packages/rate-limit/rate-limit.js:249:38)
at RateLimiter.increment (packages/rate-limit/rate-limit.js:228:31)
at app/lib/server/lib/RateLimiter.js:18:16
at saveUserIdentity (app/lib/server/functions/saveUserIdentity.js:39:8)
at ImportDataConverter.updateUser (app/importer/server/classes/ImportDataConverter.ts:245:4)
at ImportDataConverter.insertUser (app/importer/server/classes/ImportDataConverter.ts:266:8)
at app/importer/server/classes/ImportDataConverter.ts:324:26
at Array.forEach ()
at ImportDataConverter.convertUsers (app/importer/server/classes/ImportDataConverter.ts:293:9)
at app/importer/server/classes/ImporterBase.js:198:20
at Meteor.EnvironmentVariable.EVp.withValue (packages/meteor.js:1234:12)
at packages/meteor.js:550:25
at runWithEnvironment (packages/meteor.js:1286:24)"},"msg":"Meteor.userId can only be invoked in method calls or publications."} {"level":50,"time":"2021-11-27T16:36:47.926Z","pid":9324,"hostname":"fairrc","name":"Slack Importer","err":{"type":"Error","message":"object [""] is not a valid ObjectId","stack":"Error: object [""] is not a valid ObjectId
at serializeObjectId (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/bson/lib/bson/parser/serializer.js:287:11)
at serializeInto (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/bson/lib/bson/parser/serializer.js:941:17)
at serializeObject (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/bson/lib/bson/parser/serializer.js:347:18)
at serializeInto (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/bson/lib/bson/parser/serializer.js:947:17)
at serializeObject (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/bson/lib/bson/parser/serializer.js:347:18)
at serializeInto (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/bson/lib/bson/parser/serializer.js:729:17)
at serializeObject (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/bson/lib/bson/parser/serializer.js:347:18)
at serializeInto (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/bson/lib/bson/parser/serializer.js:947:17)
at BSON.serialize (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/bson/lib/bson/bson.js:64:28)
at Msg.serializeBson (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/mongodb/lib/core/connection/msg.js:127:22)
at Msg.makeDocumentSegment (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/mongodb/lib/core/connection/msg.js:119:33)
at Msg.toBin (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/mongodb/lib/core/connection/msg.js:105:25)
at MessageStream.writeCommand (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/mongodb/lib/cmap/message_stream.js:55:28)
at Connection.write (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/mongodb/lib/cmap/connection.js:368:26)
at _command (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/mongodb/lib/core/wireprotocol/command.js:120:10)
at command (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/mongodb/lib/core/wireprotocol/command.js:28:5)
at writeCommand (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/mongodb/lib/core/wireprotocol/write_command.js:56:3)
at Object.update (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/mongodb/lib/core/wireprotocol/index.js:9:5)
at Connection.update (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/mongodb/lib/cmap/connection.js:191:8)
at /snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/mongodb/lib/core/sdam/server.js:483:13
at Object.callback (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/mongodb/lib/cmap/connection_pool.js:345:7)
at processWaitQueue (/snap/rocketchat-server/1492/programs/server/npm/node_modules/meteor/npm-mongo/node_modules/mongodb/lib/cmap/connection_pool.js:468:23)"},"msg":"object [""] is not a valid ObjectId"}

Browser console:
Import_Operation_Failed /client/views/admin/import/useErrorHandler.js:1:364
l /client/views/admin/import/useErrorHandler.js:1
t http://localhost:3000/aa1d4ab72b86a914f2f370581f059e8299cabbb6.js?meteor_js_resource=true:546
a /client/views/admin/import/PrepareImportPage.js:1
Uncaught (in promise) undefined