Thanks for tracking this down. It comes down to a closing bracket being in the wrong place, so some code gets executed on any edit to any topic replies. The fix.

Line 476 of scripts/modifypost.inc.php has this:

Code
        } // end if

        $query = "
                delete from {$config['TABLE_PREFIX']}CALENDAR_EVENTS

You need to take the "} // end if" line and move that down to around line 494. Right before:

Code
        // Update the files