ADB device list empty, needed to edit build.prop
If you can't access developers options, do this first: Go to Settings -> About Phone -> click “Build Number” six times in a row. Now turn on USB debugging and give root permission to ADB.

Now ADB is working I followed the following tutorial: http://forum.xda-developers.com/showthread.php?t=1236732 and I think I put my edited file on my pc back on the phone. The problem now is that it keeps stuck at HTC screen logo. Seems there is no other option left then to restore an older version...

Edit: Guess it was better to use the original build.prop file from the cyanogenmod.zip instead of editing it. Now I have the same problem of ADB not finding my device, however this time without a booting system.

Edit 2: Through TWRP I restored an old backup, but I selected only to restore System and Boot, so I still got most of my Data :) Lesson to be learned: Before you do some changes to a system, no matter how trivial, backup everything first.