I tried the new OnlyOffice 7.3.0 and Mobile Editing ist not working anymore.
I got it working with a hack in the code itself so this would be intended behaviour.
For everyone who did that too I would not recommand an update.
As of now I am unsure if the hack can still be applied.
I will update this post as soon as I have new information.
thanks for the reply.
It seems it worked without changes.
I cannot say why I had problems the first time but after looking into the hack and realized that nothing had to be changed it worked without doing anything.
On my second nextcloud I just waited for a couple of hours until checking if it works and it worked so I changed the title accourdingly cause I do not want to mislead others.