Problem is still persisting for me. Currently on Nextcloud 21 running on Apache. It makes using the Markdown editor really difficult, because it randomly fails after a while.
I looked at the difference between the requests, when they’re working, and when they fail (due to HTTP 412). Seems the only difference between them is in the cookie. While the requests succeed, there is an entry in the cookie like this: ocfi1sfm2880=dht2tl7djsh9812aqbsubn6unb This is completely missing, in the requests which fail due to 412.