#123 ERROR:pkexec.c:138:pam_conversation_function: code should not be reached

[InfoLibre]

I added my Linux Mint MATE 18.2 computer to a Windows 2008 server, with realm. I'm using kerberos, sssd and pam_mount to mount a Windows shared folder.
When I want to launch Synaptic (synaptic-pkexec) with a user who can sudo and who is different than the Windows account, Synaptic doesn't start and I've got this error :
ERROR:pkexec.c:138:pam_conversation_function: code should not be reached

It works only if I add the option user="" in /etc/security/pam_mount.conf.xml file

Please see https://bugs.launchpad.net/ubuntu/+source/policykit/+bug/875402?comments=all