Is it possible to log in when maintenance mode is activated? If not, is there a value in the database I can change to deactivate maintenance mode?

Yes, It is possible (with backend url and admin user). I have not tested it with other users. But now it informs me wrong login. It is an annoying visual bug. I don't know if I'm doing something wrong: https://octobercms.com/plugin/support/shahiemseymor-maintenance/errors-on-backend-login-when-maintenance-is-active-and-after-also-disable