Thank you all for your help.
To complete the discription of my issue:
The problem occours with a usual 0.19.2 binary installation from your website, but my preferences where pretty old and grown over the time.
I can not say when these error exactly starts, because it occours on my office pc i rarely use for CAD.
I have only build the 0.20b to see, if the problem depends on version and it doesn't.
I have renamed the folder C:\Users\username\AppData\Roaming\FreeCAD and that fixes the problem for both versions, the standard installation and the fresh build.
At least it was my fault, because it should be one of the first things to do renaming the prefs folder, instead of taking the big gun building a beta from source and thinking about a bug - i'm sorry.
So i'll change the topic to solved and thank you very much for your help and patience.