TL;DR: DDEV v1.21.2 gives a much better error, but to fix, ddev config global --required-docker-compose-version="" --use-docker-compose-from-path=false

Current Behavior

On ddev start after upgrading to DDEV v1.21.1:

Failed to start myproject: ComposeCmd failed to run 'COMPOSE_PROJECT_NAME=ddev-myproject docker-compose -f /home/michael/www/myproject/.ddev/.ddev-docker-compose-base.yaml config', action='[config]', err='exit status 1', stdout='', stderr='The Compose file '/home/michael/www/myproject/.ddev/.ddev-docker-compose-base.yaml' is invalid because:
'name' does not match any of the regexes: '^x-'

Reason

docker-compose v1. is no longer supported in DDEV v1.21.1

In your ~/.ddev/global_config.yaml you either have required_docker_compose_version set or use_docker_compose_from_path

To fix, ddev config global --required-docker-compose-version="" --use-docker-compose-from-path=false