Have you checked for forbidden words?
I can save changes to my docs. It's either only a part of the network affected or maybe it's a network or browser problem.
I don't think forbidden words is the problem, and besides, how can I check 21 pages? I did not see this error when the document was in draft mode, but when I edit the document again after is has been published, the error pops up every 10 seconds.
Have seen this a number of times through the years - think a XHR call fails, and the exception handler pops up the error widget.
Your web session is usually still valid and subsequent XHR calls, or HTTP post/get calls, will likely work.
I just tested it again, and from what I can tell. When editing a document that has been published, it does the first auto-save, but then the error pops up every 10 seconds and auto-save stops. Perhaps the error appears instead of auto-save.