Every time i try to start Developer Mode it crashes.
I play the german version.

Someone else reported a crash with the Developer Mode executable in the German version of the game, in the topic Div 2 how to access Directs Cut.
As suggested there (starting the executable directly and reinstalling didn't help, in that case), you could try emailing support@larian.com, with a description of the problem (and any error message), game version info and the attached report.xml file created with the Divinity 2 support tool.

Other than that, you could rename the Divinity2-debug.exe file in the install folder, and try replacing it with the version for the Y-axis hotfix. If that doesn't work either, delete it and go back to the original.

I have the same problem. I think it's a problem with the shortcut. If i start directly the exe-file -> then it works.