How to find which PAM module it is?

Hi everyone,

In “another place”, I have an open thread on XRDP problems. So as not to confuse that, may I ask a related question here?

A log reports:

starting Xvnc session... pam_open_session failed: Cannot make/remove an entry for the specified session

How can I find out which pam module is complaining? I have run:
#pam-config --list-modules
Which lists 20-odd modules. None of which seem to relate to xrdp, rdp or vnc?

Any suggestions?