I had the line:

<volume group="users" fstype="tmpfs" path="mycache" mountpoint="/home/%(USER)/mycache" options="size=1G,uid=%(USER),mode=0700" />

in the pam_mount.conf.xml file. According to the man page, the attribute
"group" does not exist, I should have used "pgrp" instead, my config was
wrong.

I did not receive any warning or error even though the config file
contained invalid parameters. I checked auth.log, syslog, and well,
console output.

If invalid attributes are used, pam_mount should report them, it can
otherwise lead to dangerous mistakes.

(like in my case, cache being mounted for everyone, instead of just
members of the users group).