We experienced that with some game extensions as well as some adblock type extensions. We removed the extensions, then removed the student profile from the chromebook. When they logged back in, all was back to normal.
I saw several posts on other lists that described the same issue. Something changed in the way those extensions interact with chrome I'm guessing.
I am reconsidering my choice to allow my high school students to have some freedom over which extensions they choose to use. Just too many buggy ones right now.
-gerry