Comment 1 for bug 1530809

I had the same problem after upgrading from Mint 17.2 to 17.3 (Cinnamon and German language as in your case, difference: 32 bit in my case).

I was able to make the error message disappear by running the following two commands in Terminal:
sudo aptitude update
sudo aptitude full-upgrade

The second command asked me a few questions about differences in configuration files (e.g. /etc/issue and /etc/issue.net). I answered Y to all of them to install the version from the new package as I had not made any changes to these files deliberately.

After both commands had finished successfully, the Linux Mint Update Manager (mintupdate, "Aktualisierungsverwaltung" in German) was running fine again.