Nice to hear this, but IMO we already does this by testing the log file.
I would be pragmatic on this case and do not modify the logrotate file of roundcubemail.
I would propose to test the errors.log, if exist then we expand and start the jail, if not the jail is not created.
For what I tested if the log files errors.log is removed manually, we just warn inside the fail2ban log file, all other jails are active. When the file errors.log is created again, the jail is up without human actions.
I prefer to miss one jail, rather to put fail2ban down.
Other idea could be to modify the logrotate.d/roundcubemail configuration, manually with sed, or by a template, logrotate file configuration are built as noreplace, roudcubemail won’t replace them if modified…BUT we still need to test if the roundcubemail errors.log file exists