Unable to activate office on some accounts

We have a Server 2019 VM, with 5 user cals. 3 users have no issues and everything works fine. The other 2 are able to log into the server, but when prompted to sign in, they type in their email address, and then the box disappears.

Users are logging in to the server with AD accounts, and trying to sign into office with M365 accounts. Shared Computer Activation is on and the 365 user accounts are licensed correctly. We have seen a number of articles about the CURRENT_USER/...../Identity reg key, but we have tried all suggestions, and none make any difference.

Any other suggestions would be appreciated!