I tried to implement a non MikroTik AP, but I had to give up due to this error message.
I guess a non MikroTik AP will not work, until the RADIUS client will be able to send/receive all attributes supported by MikroTik User-Manager.

Edit: I guess that the following thread pretty much answered the question.

http://forum.mikrotik.com/viewtopic.php?f=10&t=23625