Save page does not work with Realtime WYSIWYG Editor


I could not find a bug in jira, I hope it is ok to post it to the forum.

While collaborative editing is active, saving a page does not save the page as it is. There is no error message, just the usual success message when saving a page. The page itself remains at the version it was before starting a collaborative session. If the button for collaborative editing is deactivated, saving the content works as expected.

XWiki version is 15.7. Which log would be best to check for errors?