First off, it's not magelight! A screenshot
I'm getting some VR ranks by grinding in Bloodmayne cave, and almost ever single pull I get this problem. Relogging does, however, fix the problem.
Addons are all disabled. Also, it seems that my max magicka actually does go down to what is indicated in the screenshot - when casting, I cannot cast once the bar reaches zero magicka, despite normally having enough for a couple more casts or so.
I switched out Siphoning Strikes for casted abilities and couldn't get the problem to happen again, so it might have something to do with that ability?