Looked through some older cases with that message and didn't see any obvious/simple causes. Only thing I can say to check is that the user is accessing the correct environment and using the right login (I've seen people try to use email when a different identifier is used for their account).
Apart from that wait a bit to see if others with more experience have any ideas and you can open a support case if needed.
We also have some users who are getting this error. Are there any known fixes related to this? For these users, the username is the same as their email - not sure if that's causing issues here. Any good ideas where to investigate this? I looked at the Managed server log and the audit logs, but didn't see entries for these users. I would expect to see some sort of error logs, somewhere... Unless it's somehow not recognized as an error that a user who has no environments defined (even though they really do) can't see any data. The logins should still be visible at the audit log, am I right?