The bug lies in your auth-pam.c, where you use the fork() replacement for the 
posix thread mechanisms. It seems that you never check for their availability 
and therefore never use it. 
The problem is that a pam module may be called several times, and some pam 
modules (pam_krb5* for example) rely on the fact that it can give informations 
to further calls, may it be through pam_set(/get)data or the pam environment. 
Since the pam_xx calls get called from different (forked) pids, it looses the 
address space and as a result all pam informations get lost. A force of the 
posix thread system (#define USE_POSIX_THREADS) solves that problem since the 
created thread operates in the same address space.