Unfortunately, I didn’t do a printscreen. Sorry!
But it was something like “buffer full”, or words to the effect.
Generally this kind of error (buffer related) is the HTTP server itself (nginx) hitting a request content size limit. If it happens again please take a screenshot and private message me with it. I don't think the forum software (SMF) ever generates such an error directly.
Closing this thread to cleanup the forum.
Edit: Confirmed, SMF will never report any "buffer full" like errors directly, it either came from Nginx (logs indicate it didn't) or the client's browser itself had an internal issue. All traffic for the OP's IP shows only success responses, no errors, indicating (but not necessarily proving) it was a client side issue.