Still the same problem and more of it.
I'll try to list what I have done today:
1. I had an installation of LMDE, downloaded the iso of Sparky 3.5 Mate 64-bit (md5sum was checked) and burnt it on DVD. I tried the live system, didn't notice any problem (apart from the keyboard setting which doesn't count in this respect), then I installed, formatting my HDD and deleting the old installation of LMDE.
2. Installation went fine as I have already written.
3. I restored my data and installed additional packages. This is where I noticed something strange: Synaptic still had the history of my LMDE-installation dating all the way back till April 2012 (when I installed LMDE). The history seems to be stored in some strange place, Google didn't tell me where.
4. I noticed that I can't start "users and groups", neither from menu nor terminal, both resulting in the aforementioned line in syslog.
5. I ran the live system again to try to start "users and groups" there. Impossible as well ("users and groups" appears in the panel and disappears after some seconds). So I wrote the last post.
6. I started the installed version again to doublecheck, still not possible to start "users and groups".
7. Because of synaptic's mentioned history I was afraid to have made some mistake during the install.
8. So I started the live system again and started a fresh install from there, paying attention to the formatting.
9. I deleted all the partitions of my HDD with Gparted in a first step.
10. In a second step I created all new partitions with Gparted and made it format them.
11. I booted the new install.
11a. I still can't start "users and groups"
11b. I still have synaptic's history with all the details dating back to April 2012. Strangely this history does not contain any action from the first sparky-install a few hours earlier.
11c . I had trouble creating a mobile broadband connection. I had to try six times (first Sparky install it worked the second time so I didn't mention it). It always said "Adding/activating connection failed (32) No session found for user id1000 (unknown)" (my translation, 1000 is my ID of course). Sometimes the network icon disappeared and I had to logoff and login again to get the icon back.
Any idea how to make a working system out of it?