Certificate problems are generally between a client and the specific server. All the APs do is pass the certificate information along through. Unfortunately there is not a lot to do inside of vWLAN to fix certificate issues except to get the correct certificate information uploaded (if it exists).
In your case, I believe your client was just saying they needed to upgrade their RADIUS server as it was causing the problem. A real certificate could make a difference, but without more knowledge about their particular server, there is not a way to know.
Being a client side problem, there is likely nothing you will be able to do until they resolve their server issues themselves.
Let me know if you have more questions.