Having got it again and wandered in here to start a new thread, I regretted not getting a screenshot, then it happened again whilst I was trying to start the topic (and then also afterwards whilst trying to Preview, a couple of times...) so screenshot attached, not that it's very informative beyond the basic detail of the 504 (and a few of the other tabs I have open, and device notifications pending dealing with, but nothing exactly embarassing...).
ETA: Forgot to say, I've received this upon posting a message then (when I've checked) the message was posted, suggesting the problem is only in the daemon response to my submission. But I also had to submit the last Comic Thread starter I made twice because the first attempt failed utterly, so there the submission failed to even be (fully?) received by the back-end, so far as I can deduce from the minimal data. Which probably thetefore doesn't help.