The AcoustID Settings page under Fingerprinting will save whatever you put in the field without stripping leading or trailing whitespace. This is a problem, because it then tries to use the erroneous key—whitespace included—when uploading fingerprints. This does not work and the upload will fail

Code on GitHub

def save(self):
    # ...
    config.setting["acoustid_apikey"] = self.ui.acoustid_apikey.text()
    # ...

Picard should strip any leading or trailing whitespace when saving the key. (A cleaner user experience would be to also strip whitespace when the field is unfocused)