20vek,
As its working with PAP and not working with MSchap there could be two reasons;
1.] Mschapv2 is not allowed on ACS >> access policies >> default network access >> allowed protocols >> Allow Mschapv2
Please attach the full failed attempts from ACS in your next reply.
2.] You are running ACS 5.0
This was a bug in earlier release but actually applicable for PAP
CSCta05652 Active Directory operation has failed because of an unforseen error
HTH
Rgds,
JK
Do rate helpul posts-
~Jatin