@Michael-Huhn
Thanks for your reply.
I have tried to simply click into the address bar and call up the page again, with no success though - the login page gets reloaded then, too.
Turning off auto-fill option in the browser doesn't help either ways, the message i-doit system error: Login failed: CSRF-Token mismatch!
does not disappear.
Any suggestions?
Best regards,
Ronny