We are facing exactly the same issue.
Has this been reported to Mantis? I couldn’t find anything relevant there.
Is there a suggested fix? We are currently updating our usernames to longer ones, in order to avoid the issue.
If you confirm that there is no associated bug report yet, should we create a bug report on Mantis?
Yes, we can replicate the issue. You can follow the progress here:
The simplest way to fix this is to edit the phplist_admin table in the database. Just add a numeric suffix (or something) to the loginname and namelc fields. That’s what I did.