The ACVP server seems to reject any request in which an AES-XTS data unit length does not match the payload length, even when dataUnitLenMatchesPayload is set to false. The following message is returned:
[ {"acvVersion": "1.0"}, { "error": "Validation error(s) on JSON payload.", "context": [ "ACVP-AES-XTS-2.0: Unable to build test cases where multiple data units fit within one payload; this may be because the DataUnitLen and PayloadLen are overly restrictive" ] } ]

I've gotten this response to the following requests:

1. DataUnitLen = [512], PayloadLen = [256, 512]
2. DataUnitLen = [1024, 4096], PayloadLen = [65536] (this is the example found here for AES-XTS Testing Revision 2.0)

The full requests sent to the server are attached as Ex1.json and Ex2.json.

Ex1.json
Ex2.json