I'm getting this error occasionally this morning, where loading a thread will result in an otherwise-blank page with "File Not Found" written.
Similarly, it will occasionally happen when choosing a reaction to a post - this time, the pop-up will be in the bottom left, similar to the pop-ups that arrive with other load-related issues (can't-handle-your-request-right-now, oops-overloaded, etc.).
intermittently and seemingly randomly
It apparently did submit because when I reloaded the page and resubmitted, there were 2 of them.