IAS not authentication or logging requests

I have a strange situation now.
We had a crash on one of our DCs for a few weeks ago. THis server has now been replaced with a new one (new name and IP).
A few weeks after this exchange, the authentication of computers in the WLAN stopped working. We are running a Cisco WLC4402 Wireless Lan Controller that authenticates computers by RADIUS (MS IAS). I don't know if these two situations are related, but anyway...

Now I see that the WLC is sending requests to IAS. But the IAS does not answer. The request is not logged in eventviewer.
WLC logs : "RADIUS server 192.168.56.22:1812 failed to respond to request (ID 203) for client 00:21:6b:9a:81:8a / user 'unknown'.
Sometimes the request gets logged in the IAS-logfile, but this is about 2-3 out of 4000....
And it seems like the requests that get logged are the ones that is "easy" to reject (user does not have dial-in permission). The requests that have valid certificates (and therefore should be accepted) are not logged.
Now and then the IAS-service stops.

I have tried to set up Network Policy Server on a Windows 2008 server, but the problem is the same....

Coud this be a problem with AD?
To me it seems like the IAS gets a huge workload, and does not manage to look them up...?