It's unclear how to fix this, since it would need at least decoupling the decryption from signature verification, which doesn't appear to be so easy to do with just gpg - I couldn't figure it out. There's still a bug open upstream though, so my suggestion would be to add as much debugging information to that bug report so that someone can look into the issue and try to fix it.
Note that there are already possible workarounds. One of which being to manually download the public key for the user sending the email; since the error will only show up if an encrypted email is sent and no public key is available. It's also possible to send emails in different formats (not pgp-mime) which will allow evolution to display the email properly.
To manually download public keys, you can start "seahorse" (the Passwords and encryption keys application, search for keys by name or by ID (with the ID provided in the gpg message), then use the import button once the correct key is selected.
It's unclear how to fix this, since it would need at least decoupling the decryption from signature verification, which doesn't appear to be so easy to do with just gpg - I couldn't figure it out. There's still a bug open upstream though, so my suggestion would be to add as much debugging information to that bug report so that someone can look into the issue and try to fix it.
Note that there are already possible workarounds. One of which being to manually download the public key for the user sending the email; since the error will only show up if an encrypted email is sent and no public key is available. It's also possible to send emails in different formats (not pgp-mime) which will allow evolution to display the email properly.
To manually download public keys, you can start "seahorse" (the Passwords and encryption keys application, search for keys by name or by ID (with the ID provided in the gpg message), then use the import button once the correct key is selected.