possible bug

Okay, so I'm having an issue where specific content in a specific forum is triggering a user-sided error without an admin error log. One of my members is a moderator in a specific sub-forum. They attempted to create a new thread (with a prefix) but received a server error, not letting them...

Basically... The video player on the page below doesn't want to fill the page, it's tiny. Can't seem to figure out the reason for the player being so small, ideas? https://veganforum.co.uk/media/2015-xmas-competition-winner.164/