looks like a timeout error, 503 (service not available)
[08/Oct/2022:13:37:44 +0100] "POST /forum/posting.php?mode=reply&t=463 HTTP/1.1" 503 3425 "-" "Mozilla/5.0
Out of curiosity, what does the number after the status code mean? In this case it’s 3425.
Now to track down the reason for the timeout, PHP, Apache or PHPbb.
Thank you Joulinar.