EmbedVideo uses a deprecated function for messages:
Use of MessageCache::addMessage is deprecated. [Called from EmbedVideo::setup in /usr/local/www/wikifur/prod/extensions/EmbedVideo/EmbedVideo.php at line 81]
This has been removed entirely in trunk/1.18:
According to a comment there, the appropriate method is now through $wgExtensionMessages or the LocalisationCacheRecache hook.
Thanks for the information. I'll take a look at the replacement options and try to update the code.