Hi,
I've managed to solve this problem. The problem was with Active Directory Legacy Mode. Apparently it does not support EAP-PEAP with MS-CHAPv2, although the IC documentation stated only that the native Active Directory mode is RECOMMENDED for 802.1x with MS-CHAPv2. There is no sentence about Legacy mode not supporting it at all.
Is your AD a 2008? Pretty sure I've done that before "new" mode was implemented, but on a 2003.
Yes, it's AD2008.