Thank you HCP.
It was a real pain, but thanks to your scripts I was able to recover a fraction of a BTC that was sent to my Mutlibit 0.5.18 installation. And I couldn't access the wallet file anymore due to the error message
Could not load the wallet file "C:\..." The error message was "null"
I had to use the full command line to Python (so c:\Pytohn27\python dectrypt_..." instead of just "python") to make it work. I got the decripted keys, wrote them off the screen (copy-pasting didn't seem to work in MS-DOS) and sweeped them into Electrum.
So thank you again for your help!