You receive the following error message when you try to synchronize with Active Directory:
Windows does not allow simultaneous connection of different users on the same computer to the domain controller.
It is possible that your current computer is already connected to the server controller as UserA (for example, Windows Explorer might use a shared network folder on the server). If you now try to connect to the server as UserB, you will receive this error message. Use the UserA account to avoid the problem.
Another possible case:
If you try to connect to the ComputerA server as UserA, but UserA is already connected to the ComputerB server. You may first need to disconnect from UserB on ComputerB or, alternatively, use a different account.