Internal Error is very generic in error. I hope that you had your Logging set to Full, if not then you wont be able to see the exact reason in the debug logs.
You might want to check,
\CSAuth\Logs
And check the debug log when you got the internal error for a particular authentication attempt.
Also, what kind of authentication was failing ? Was it PEAP/EAP-FAST with inner method as MSCHAP machine authentication, then it could be something related to,
CSCsq96755 : ACS needs manual restart to recover machine authentication
Then go for Patch 5 for ACSv4.2
Regards,
Prem
Please rate if it helps!