Maybe Jeff has finally took his vengeance on all of us with his giant S?
Those nasty errors happen on page load then whenever I type something in the tag editor. Naturally, they cause I can't choose tags because no tags appear. I can still edit tags and post new questions by manually typing the tags.
Using Chrome latest stable version (30.0.1599.101 m) but doubt it's browser specific.
As a temporary workaround for those with weak memory, you can go to the tags page and the search/filter in there does work.