I think it's 100% the goalkeeper's mistake, no one else is to blame. The one who makes the decision is responsible, he made the worst decision possible.
By the way, I heard that in pre-season games Monaco beat Barcelona 3-0, so in some sense you can say that Barcelona is progressing
This game looks like an accident rather than a logical outcome, so I hope everyone (including the bookies) continues to overrate Barcelona.
Regarding the communication error or maybe it was more Ter Stegen's fault for something like this in the game there are always things like this and of course this should be used as a situation where they can improve even more because this is not the first time mistakes have occurred in the Champions League because if we remember last season they also did the same thing when meeting PSG in the Champions League which in the end made a loss for themselves.
The second week they will face Young Boys should be a calm before the match against Bayern returns with an interesting situation between Barcelona, Flick and Bayern.
I can't say Monaco had a good performance because of beating a team like Barcelona in the ChampionsLeague, they really won the game but it doesn't mean Monaco is a good team, they were playing against Barcelona while Barcelona had one fewer player for the whole this game and this was effective for them to beat Barcelona.
Since Barcelona got good results in La Liga, I think time after time they can have better performances in the Champions League too.