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I have found this error on a PHPBB forum:
PHP Fatal error: template->_tpl_load_file(): File /message_body.html does not exist or is empty in includes/functions_template.php on line 63\n

When searching for this file I could see it exist at ./styles/subsilver2/template/message_body.html
and the subsilver2 is my only style.

After checking access log, i could find the request that is causing this:
/posting.php?mode=reply&f=123&t=1234

After opening that forum and topic i could see no such topic exist. So I am guessing PHPBB just does not handle this "edge" case properly.