I am afraid that Quizlet has done something to their site that has broken our app and the search functions no longer work correctly. I have tried to fix the problem but it seems like the responses I am getting from the Quizlet site are not simply not correct. I have tried to contact them multiple time to resolve the problem, but they have not returned any of my emails. It appears that now that Quizlet has it's own app, they are no longer interested in supporting 3rd party apps. It is likely that I will remove Quizlet from the list of supported sites in the next release.
If you are committed to Quizlet, my best advice at this point would be to switch to using their app.