While I do not have voyager on the phone, trying it out on the web, it seems like voyager does not really have custom error messages for errors returned by the Piefed API (or at least in my case)
I got a 400 status code with incorrect_login error message despite seeing a "Connection error, please try again.".
It's possible that it is something similar.
If you are also using it on the desktop, try checking the network logs.