No worries. During our play session, this bug did not appear again. To me it seems like some random occurrence.

I was not able to pinpoint exact steps to reproduce it either.

Thanks for the info, I updated with the correct one (I loaded the save to confirm that it was valid).

What did you do?
1. Join a friends game via Steam
2. Friend stops the game
3. Click on "save" in the resulting "Server ended" dialog

What happened?
During saving, Factorio crashed and showed an error: "Unexpected error occured".
Factorio saved the game successfully, the save game is not corrupt ...