408 is http code for "Request timeout" which is why I lean toward the web server, not peertube, being the problem. However, I am no web socket or server guru, so where to start tracking the cause is a little unclear to me. The art of problem solving is asking the right question, and I haven't figured out the question.
I have a thought now, though: check the URL returning the timeout and see what my server settings are for that specific path....