Most likely the errors have something to do with not finding the required public keys with which to verify against, although you should only need one developer's public key and one verified SHA256SUMS file in order for the verification to work successfully.

If "hkps://keys.openpgp.org" doesn't work for you (like with some Windows users), use "hkps://keyserver.ubuntu.com".
[/quote]

I don't think most keyservers are working when you try to import a new key from there, so I would opt with directly downloading and importing it instead.