Thank you for the fast answers.
To the first answer: As i already wrote in the first post, I tried to load the restored backup with 16.22, which is not a downgrade anymore, and it still did not work.
The file i did upload was never manually manipulated by me. What exactly of the content is broken ...

Today i did a downgrade from latest Experimental Release 16.22 to 16.21 to be able to play on a multiplayer server. As soon as i did load the multiplayer map (no mods, then running on 16.21) i got the following error:
FailedToLoadBlueprintLibrary.PNG
After this error my blueprint library was ...