Unable to access to Plesk under admin: Another user with the same username (admin) is already logged in to Plesk

Symptoms

Unable to log into Plesk with administrative account. A following warning appears:

Error: Unable to log you on. Another user with the same username (admin) is already logged in to Plesk.

Cause

Allow multiple sessions under administrator's account option is disabled.

Resolution

1. Connect to the server via SSH / RDP.

   Note: if direct SSH/RDP access to the server is not possible, contact server administrator for further assistance.

2. Run the command below:

   # plesk bin admin -u -multiple-sessions true