LightDM - Jul 2018 - Output from 'groups' is suddently different to /etc/group

Hi all:

Since a couple of weeks, after logging in to my Kubuntu 16.04.4 system, 
group membership is incorrect. My user account rdiez is only member of 
the rdiez group.

However, if I log in with a text console (without X), it works properly.

You will find the full details here:

https://lists.ubuntu.com/archives/ubuntu-users/2018-July/thread.html

After some more research, I think it may have to do with LightDM. But I 
know very little about login managers, PAM, etc.

Could you please give me some pointers about how I could troubleshoot 
this issue?

Thanks in advance,
   rdiez