The alike error appears during the attempt to remove the database user from Plesk Panel:
Delete database login failed: Could not drop login 'user' as the user is currently logged in. at (VADOConnection::execute line 68) at execute console command --del-login(vconsoleapp::start line 97) at execute "C:\\Program Files (x86)\\Parallels\\Plesk\\admin\\bin\\mssqlmng.exe" --del-login "--server=localhost" "--login=sa" "--password=*" "--user-login=user"(vconsoleapp::run line 107) (Error code 1)
The same user is currently logged in the MSSQL Management Studio and performs some actions.
Repeat the user removal from Plesk Panel. In fact it is being deleted from the very first attempt, but an entry leaves in Plesk database.