Hey Jan, so I''ve tested with the instructions su sent me. In total I was trying to add more than 80k words to the doc and I was able the reproduce the error. This is an expected behaviour of the writing room, when a document is way to large or contains too. many items it starts to break.
It's similar to what happens if you add to many elements in a group field. Look at this thread
So the workaround for this is the same as the linked thread above: You need to separate this file into smaller documents so that it becomes more manageable.
I hope this helps you, and if you need anything let us know