This is a feature recently introduced in Dynatrace Managed (v190):
Your cluster administrator has set the restriction to number of concurrent logins. You have to reach to your cluster admin to either close your sessions or increase the number of concurrent sessions.
Thank you for your comments.
I tried to set up to concurrent logins with 1 user and 1 admin and closed browser.
Then login in it again but still same messages above.Do I have to wait for 15 min? Or
How do I login in it again? I should reboot the system?