[SOLVED] After searching the whole evening I found an old reply from Chris Dobbs - Steinberg US Support which helped:
Re Cubase 7.5 freezes when trying to edit midi parts
Postby Chris Dobbs » Wed May 07, 2014 352 pm
I would suggest clearing your App Data to give 7.5 a fresh start
First, please shut down Cubase. Then go to the Start menu, All Programs, Cubase 7.5, Cubase 7.5 User Settings Data Folder. In the top address bar, you should see “App Data (roaming) Steinberg” Click on Steinberg at the top. This will put you in the Steinberg app data folder. Trash any Cubase or Nuendo folders that you see there. This will trash your key commands, preferences, sound card settings, etc. Hopefully this will also should clear up the problem you are having. Restart Cubase. You will need to reset your audio outputs (Devices, Device Setup, VST Audio System).
If the problem persists, are you running in 32 bit or 64 bit
Steinberg US Support