It appears that since the last update (2.101), MySQL/MariaDB users are being restricted to 16 chars by default. In earlier versions, this was present in /etc/webmin/virtual-server/config
mysql_user_size=80
Now, it no longer seems to be present. Re-adding that resolves the problem however.
I didn’t know that, I notice virtualmin will stop at 16 character automatically when creating say wordpress. Really this shouldn’t be a issue unless for some reason you want a custom name.
Well its in the forum if anyone needs a larger name, seeing its default for MySQL/MariaDB I can’t see this being a fault of virtualmin. Maybe Joe or one of the other staff may respond.
I ran tests with Debian 12 installed from the link you shared and clean Virtualmin GPL install, and I could not reproduce this issue. For me mysql_user_size was set to 128.
Did you make any virtual server restores that included Virtualmin configuration?