I unfortunately ran into the same problem: After the transfer from another account, the app groups don't seem to work anymore.
This is the error message I get:
An Application Group with Identifier 'my.app.identifier.group' is not available. Please enter a different string.
Were you able to resolve the issue already?
With best regards
I'm going through the same issue.
Were you able to solve this somehow?
Would deleting the group on the previous account and re-creating it on the new one keep users data intact when pushing an update with the new account?