When I receive an OpenPGP encrypted message, with encrypted subject, I get a blank message that shows "Subject ..." and no body. The OpenPGP icon above the message window shows a lock with a red slash through it. When I click that icon I get a dialog "Message Security - OpenPGP" which is what I expect. However, it also includes an error: "Message Cannot Be Decrypted There are unknown problems with this encrypted message" The error log shows: rnp_op_verify_execute returned unexpected: 301989892 2 RNP.jsm:1151:17 The only relevant reports I could find were specific to Windows and fixed by the user going to the 32 bit version of Thunderbird. I have not tried that because it looks like an integration error with RNP, and googling RNP and Yubikey suggests a smartcards are not supported by RNP. If I get encrypted content Thunderbird correctly invokes the pinentry program, and decrypts the message just fine. Extracting a portion from a working non-OpenPGP message looks like this: Content-Type: multipart/alternative; boundary="000000000000c3a59f05e3dd7e79" --000000000000c3a59f05e3dd7e79 Content-Type: text/plain; charset="UTF-8" -----BEGIN PGP MESSAGE----- ...