User Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.102 Safari/537.36

Steps to reproduce:

I am using Thunderbird on Fedora 32.
A recent update brought Thunderbird 78.3.1.
After starting this updated Thunderbird the PGP encryption behavior changed.
Instead of the enigmail plugin, Thunderbird uses its built-in solution.
Unfortunately, the new built-in solution cannot decrypt a huge amount of my daily received emails, which used to simply decrypt with enigmail.
Note, that I can tell external people to use better PGP tools, but some have company policies that simply do not allow them to do that. Telling people to use a "better" tool, because my tool cannot decrypt, will not work.

The exact steps to reproduce are:

• receive external partially encrypted email
• Thunderbird states: "Only a subset of this message was encrypted using OpenPGP..." -> Click on "Decrypt".

Actual results:

Instead of decrypting the email, I see the encrypted email contents ("-----BEGIN PGP MESSAGE-----").
See also attached screenshot.

Expected results:

Show the decypted message (just like enigmail did).

Can you provide more info: https://wiki.mozilla.org/Thunderbird:OpenPGP#Debugging_.2F_Tracing

The error console gives me the following:

TypeError: this.returnStatus is null
mimeVerify.jsm:565:7
    onStopRequest chrome://openpgp/content/modules/mimeVerify.jsm:565
    onStopRequest self-hosted:850
    sync chrome://openpgp/content/modules/cryptoAPI/interface.js:51
    onStopRequest chrome://openpgp/content/modules/mimeDecrypt.jsm:559
    onStopRequest self-hosted:850
rnp_op_verify_execute returned unexpected: 268435457 RNP.jsm:876:17

https://hg.mozilla.org/releases/comm-esr78/file/tip/mail/extensions/openpgp/content/modules/mimeVerify.jsm#l565 - but I guess the problem is whatever caused rnp_op_verify_execute to fail

The unexpected return value is 0x10000001.
Do you have some ideas what these bits mean or at least where to look up that information?

User-Agent Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.3.3

When receiving an external encrypted email from Thunderbird 68 and enigmail, thunderbird does not decrypt the message but shoes ("-----BEGIN PGP MESSAGE-----"). See also attached screenshot. Sending an encrypted email to the same external recipient works and is correctly decrypted at the other end. Thus, keys are known.

Error console returns: rnp_op_verify_execute returned unexpected: 268435457

Christoph, is your problem solved with Thunderbird 78.5.1 ?
If you are using allow_external_gnupg then maybe you were affected by bug 1675939 ?

I just tested with TB 78.5.1 and the error persists.
I can also confirm, that allow_external_gnupg is set to false (did not even know about that before).