group membership missing after update

[Markus Grunwald <debianuser.thegrue@spamgourmet.com>]

Hello,

after many years without any (debian testing related) trouble, I need
your help again: After my last safe-upgrade, I'm not member of any
groups when I open a shell...

-I login to my i3 windowmanager
-open a console
-output of "groups" is "markus" - and that's all :s
-after a "su - markus", or ssh localhost, or switch to the text console,
all is normal...

An "strace groups" shows, that /etc/groups isn't even touched...

I have no Idea what wrong there and hope for your help.
TIA
-- 
Markus Grunwald