I've gotten this twice after submitting a post, once yesterday and once just now:
Top of the page:
[phpBB Debug] PHP Warning: in file [ROOT]/includes/functions_messenger.php on line 1426: stream_socket_enable_crypto(): SSL operation failed with code 1. OpenSSL Error messages: error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed
Body of the page:
SQL ERROR [ mysqli ]
MySQL server has gone away 
An SQL error occurred while fetching this page. Please contact the Board Administrator if this problem persists.
In both cases, my post went through and showed up, so whatever problem this is, it wasn't a problem writing my post to the DB.
ETA 3 times; it happened again when I submitted this post.