I've added 3 accounts to the app and 2 work fine. My lemmy.ml account returns Value <html> of type java.lang.String cannot be converted to JSONObject and since this only happens in the lemmy.ml instance I can assume its server related.
I am, however, able to use the webapp with this account just fine and lemmy responds relatively fast.
This is due to poor error handling in the API client code, triggered by the server returning some sort of error. There's an open issue but it hasn't been taken up yet.