Have been testing 8,5 pro trial for a week now
but when I double-klick on a midipart for looking at it in the Key-editor
the program locks and I can´t see anything in the Key-Editor. 
This works ok in my 8.0 Elements
so what´s the problem? 
Tom
There are several possibilities that can cause this behavior. A good way to start is this, because a lot times things like these are caused by corrupted preference files:
Cubas 8 Elements: For me the renaming of the programm Folder worked - subito. It is described in the steinberg document linked to this thread - Citation starts:
Initializing preferences
Corrupted program preference files can bring your Steinberg program into all kinds of troubles, particularly after installing an update.
Fortunately, refreshing the preferences is quite easy:
Close all programs.
Open the ‘Run’ command prompt by pressing the Windows logo key and R on the keyboard simultaneously. The Windows key is located between the keys Ctrl and alt.
(To open the command prompt on Windows XP select ‘Run’ from the Start menu.)
In the command line of the ‘Run’ window, enter this path: %appdata%/Steinberg
Locate the folder which is named the same as your Steinberg program, for example ‘Cubase 8.5’.
Rename it (for example, from ‘Cubase 8.5’ to ‘xCubase 8.5’) in order to hide it from your Steinberg software. The folder will only be hidden safely, if the change will be done before the original folder name.
If you find folders of previous versions of your program, make sure to hide or remove them, too.
Next time you start your Steinberg program it will create a new folder with fresh preferences and default settings - which hopefully fixes the problem you’ve experienced.
Please note: Make sure to hide or remove preferences folders of all installations of your Steinberg program. Otherwise the program would try to comply with them when creating fresh preferences. And exactly this could be the source of errors.
ACHTUNG: Before you use this solution!!!
The key-editor worked again - but many vst-instruments are not longer assigned. So all my old project do not work anymore - years of work.
I tried rename the folder but the projekts will not work any longer. For example: My Komplete Control Data ist not any longer assigned. It would take weeks to fix this again.
Although my version is quite old I think I will leave cubase finally, because the disappointment is huge. After this post I will start an test-installation of reaper.
if anybody has a solution to repair the old vst-assignments and the project-related files - I would appreciate your help.
Reset of preferences can not de-assign any vst instruments in projects.
Maybe you have additional plugin paths you haven’t added back into the prefs or those instruments are not available on the computer any more.
Grim, you are right. It was a problem of an simoltaneously update of NI Plugins which somehow changed the folder’s path. I had to reassign everything and now everything is fine againe! Did not expect it - should have been more patient. Thanks for your message.