I have tried to import a public key of someone into enigmail. The import failed silently (nothing absolutely happened, not even an error). I tried key from file and from clipboard.
Then I used gpg --import on the file and the key appeared in gpg --list-keys but untrusted. So I used gpg to trust the key completely ("uneing." in German).
Enigmail now shows the key but with a Red box. I can not select the key and use it to encrypt messages.
gpg --list-keys output
pub 0E/xxxxxx 2018-03-07 uid [ uneing.] xxxx <firstname.lastname@example.org>
When I use key-management in Enigmail to validate the key via right-click "Key properties" and Certify button, I select "I have checked very carefully" and an own key for signing. After clicking Ok again nothing happens, key is still "invalid" and no error appears.
Enigmail-Verison: 2.0.10 (20190324-1211)