As I stated in the message title, this problem occured as soon as the maximum number of user accounts was reached.

The message said:
"Your server's license has expired. The server does not authorise anymore the sending/recieving of emails, but you can still access the data from the local client"

The server's license is not a temprary one, but a fully fonctionning 250 users user accounts file.

Does anybody have an idea about what just happened?

By the way, after upgrading the license to 300 user accounts, everything got back to normal.