ugg. Trust Yoast to be the one cause an issue. Can you turn on your WordPress debug.log flags? If there's a crash happening there should be a message in the debug.log file. Or, if it crashes all pages then you should have gotten an email automatically - WP sends out an email with crash details when a major error occurs.
Yes, we were able to duplicate the error and quickly band-aid it to make sure sites aren't crashing. Not sure though what all the changes they've made mean for us - we were turning some Yoast things off in the core plugin to make sure that they didn't stomp all over some SEO items specific to the forums. If it turns out that they've botched backwards compatibility we might have to temporarily declare Yoast a problem until we can figure out what they did and redo all that work. But for now it should no longer crash the forum.