what instance were you trying to sign into? because i just tried it with lemmy.ml and it works. also could you also say the version of the app? (you can find it in settings->about lemmur) in general there's some stuff in the app that might not work because we haven't updated the app to work with the latest changes to the api yet. but basic stuff works, that's something that i'm sure of
edit: also wanted to note that this is not an official app, we are independently working on it as a hobby. the official android app is being worked on but idk how far are they with the project