Just wanted to let others know how the problem was solved (thanks alot to Lars and Mat):
I had to install the latest version of pam_radius_auth from http://www.freeradius.org/pam_radius_auth/ (ftp://ftp.freeradius.org/pub/radius/pam_radius-1.3.16.tar
) since my version was 1.3.15 where the pam_sm_acct_mgmt() function was missing.
Now it works well.