Hello,
sorry, I am totally new at linux and onlyoffice and maybe my question is a little bit dump, but… how is it possible to accept (all) changes? I 
Hey LeChuck!
You need to select a change / use “Zur naechsten Aenderung” to select a change. Then “Annehmen” is not grayed out anymore and you can use the small arrow below “Annehmen” to choose “Alle Aenderungen uebernehmen”.
I’ve tried to reproduce the situation on the screenshot, but I failed.
But in general, it looks like @nilskamm is right.