I had this error previously, I think it occurs when you don’t use the same key for connecting the client/server so try checking that. It is the key you used in the android rpc app, in the the example you find it at tracker.request(key, priority=0, session_timeout=60).
Personally I don’t use any key (so I empty the field in the android app and put an empty string in the code). The key is useful when you have multiple devices and you want to distinguish them.
PS: you also find the devices keys in used when running python -m tvm.query_rpc_tracker in the Queue Status section (when using no key you just the output of the other columns not aligned).