Sorry if this is too basic but have you deleted the cache and force-closed the app before trying again? Or even the storage, if you don't have any other accounts on it to worry about.

If you don't know what all that means, then I suggest uninstalling the app (if you don't have any other accounts on it), rebooting your phone, and then reinstalling the app to try again. It's overkill, but should offer the best chance of working unless somehow that connection is being blocked by something strange and more specific to you.

I don't really have a Piefed account, so I just typed in random stuff to see if my prediction was correct.

You should check the sent data to see if the password or username you typed in are really sent the way you typed them in (as rimu pointed out, it could be that there are some spaces around the username or the password for whatever reason).

While not something I encounter alot, sometimes using the autofill function of my password manager (Bitwarden) somehow causes the password to not be accepted by the server. Try manually copying and pasting the credentials. If nothing works, I guess open a bug report at voyager repo.

I doubt this has to do with Piefed's API.

Track progress https://github.com/Blorp-Labs/blorp/issues/330