google search led me to a previous problem like this, with the php version being at fault.[phpBB Debug] PHP Warning: in file [ROOT]/ext/paul999/mention/event/main_listener.php on line 359: sizeof(): Parameter must be an array or an object that implements Countable
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions.php on line 1823: Cannot modify header information - headers already sent by (output started at [ROOT]/includes/functions.php:3037)
the curious thing is that it happens when the post DOESN'T include the mention tags. if the post has it, the post goes through and no error message. if it doesn't have the tag, then it sends to the error message, but the post is sent through.
I'm on version 3.3, php 7.2