I'm testing the new version, but when I run the update I have this strange message at the end of the process: Catchable fatal error: Argument 4 to mysqli_connect() must be of type ?string, bool given in tmp/ispconfig3_1_roundcube/install/lib/mysql.lib.php on line 204 I verified that everything is ok, the system works but I do not yet understand why this error!
what version of "3.1" are you using there? I believe there were some mysqli errors fixed along the way, and I've not seen any for a while; I just updated to git-stable (via ispconfig_update.sh script) right now and don't see that. You may just need to get a newer "3.1"?
I don't have an environment to test that 3.0->3.1 update, but I'd grab the latest tarball from git and try that. If it still happens, it's probably worth filing a bug report at https://git.ispconfig.org/ispconfig/ispconfig3/issues
