Sofar I have uninstalled both Freecad versions (0.16 and 0.17) and reinstalled version 0.17, but the problem remains.
This usually doesn't help because you haven't modified the installed files but something else has changed.
Now I have run into the problem, that FCStd files are not listed among the supported formats of Freecad.
I can save as FCStd, but when I try to open the same file (or others) from Freecad it is not shown in the list, and when I choose 'all files' and select the one I want to open I get the error message "loading the file xxx.FCStd is not supported".
What does work for now, is double click on the file in explorer, after saying "open all files of this type with 'Freecad'".
Also the Freecad version 0.16 wouldn't start anymore, showing the error message "'ascii' codec can't decode byte 0xc3 in position 211: ordinal not in range(128)".
When starting Freecad 0.17 I now get the message in the bottom left : "unknown command Std_AddonMgr" - the addon manager was available when I first installed, yet now it is gone.
This sound very weird. I would try two things:
1. Rename your macro directory (check your preferences for the actual path)
2. Rename your Mod directory of 3rd party modules (%APPDATA%\FreeCAD\Mod)
There are no fancy characters in the directory path or in the install path of FreeCAD: "E:\Program Files\FreeCAD 0.17\bin\FreeCAD.exe"
At first I had it installed on the C: drive, but during my attempts to solve the problem I tried installing it on E: instead - made no difference.
And what about your user name? Does it include things like umlauts?