Ubuntu+NC+Collabora: Can open Documents but not edit

Hi, I just posted this problem in German, please don’t mind.

I have installed Collabora without Docker (using https://github.com/husisusi/officeonlin-install.sh/archive/master.zip). I can open documents, see and navigate through them but I cannot edit them at all. This means, when I open the document, I can scroll etc. but as soon as I try to change something (e.g. press one character to add it) there apears “xxx left” and the document is reloaded unchanged.

My System: Ubuntu Server 16.04.4 LTS, NC 13, Collabora 2.0.8, Apache/2.4.18.

Hint: I guess, that sessions are not saved properly so that with each request the server is creating a new one which is not saved either.

At least I can read following line coming with each reaquest:
“[ docbroker_004 ] ERR Invalid or unknown session [004a] to remove.| wsd/DocumentBroker.cpp:1069”

Does anyone know something about it?