Thanks for your answer Tironei,
Because of your comment, I decided to reset the entire board.
This fixed the problem for me and I was able to play against chess.com players for the first time. 
Before I just tried the option ‘Clear Credentials’ on the Chessup app, but this didn’t work.
Hi,
I had a very long wait today as well. I then canceled the game request, but a game started anyway… And then I lost, because I apparently quit the game. Which I didn’t… So, lost a lot of rating points. Mmmm…
Still getting this error with latest update
I would suggest resetting the whole board 
How did you lose elo? When a game starts and you haven‘t made a move you cannot lose. The only thing that can/will Happen is that the Game gets cancled. Than it‘s ±0 Elo.
Did you use chess.com?
I did one move, my opponent two moves. But you are right. I looked at the wrong game. I did not lose points. But chess.com said that I left the game. Furthermore, the game did not show on the screen of Chessup. The moves of my opponent appeared on the board… Strange…