If you can’t sign/decrypt with a YubiKey and Thunderbird/Enigmail you might want to add
--pinentry-mode=ask to the “additional parameters for GnuPG” in the Enigmail configuration
After setting up all the cool Encryption stuff using a YubiKey I was so happy that everything worked.
And then I set up using the YubiKey for SSH as well as described in the documents I linked in the last blogpost. It took a reboot for everything to work out as I wanted it, but I was happy. Until I wanted to send a signed Email using Thunderbird/Enigmail.