licence error Acrobat XI Pro

am stil using this version, because there is no decent alternative. But i am getting a licence error, the licence is already in use by a maximum allowed computers. You need to deactivate at a different computer.

But i dont know where it has been activated, when i check my account i see only one pc in use. So i contacted support, but that didnt help.
Anyone who knows a sollution for this problem?
Thank you.