User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:92.0) Gecko/20100101 Firefox/92.0

Steps to reproduce:

I migrated my thunderbird from 68 with Enigmail and GnuPG to 91.1.0 64Bit with all OpenPGP keys migrated (I tried thunderbird 71 too).

View a OpenPGP message I send to another person.

Actual results:

The email viewer shows at some OpenPGP encrypted messages I send to another person only the encrypted data and an error that there is a unknown problem with the encrypted email.
If I reply to such a message I get a reply with the decrypted email.
I have one small message with remote public key I used I can give private to the developers.

Expected results:

The email viewer should show every time the decrypted email.

With Thunderbird 68 and Enigmail/GnuPG I have no problems with viewing all emails.
Thunderbird 91 is blocked a short time if OpenPGP is used.

I heard today that the receiver has the same problem with this emails he got from me.

Are you talking about PGP/inline? Thunderbird does not support sending that. We only use PGP/MIME.

PGP/Mime

If I reply on my send email, I see the clear text like I showed at the screenshot.

Thunderbird 91.4.1 (64-Bit) has the same bug.

I'm sorry that this bug never got any attention.

You had not provided us with a sample email, which made it difficult to analyze.

Your screenshot looks like PGP/INLINE, not PGP/MIME. An attachment with a sample message would help.

Looking at your screenshot, I see something unusual, which could be responsible for the failure. After the ---BEGIN header, a comment and charset line may follow, but after that, there's usually a blank line. Your screenshot shows text =C2=A0 at that position.

If I had a full example email, I could check if the structure and encoding are valid or not, and what happens when we try to process it.

(In reply to Kai Engert (:KaiE:) from comment #6)

I'm sorry that this bug never got any attention.

You had not provided us with a sample email, which made it difficult to analyze.

Your screenshot looks like PGP/INLINE, not PGP/MIME. An attachment with a sample message would help.

Looking at your screenshot, I see something unusual, which could be responsible for the failure. After the ---BEGIN header, a comment and charset line may follow, but after that, there's usually a blank line. Your screenshot shows text =C2=A0 at that position.

That makes this a duplicate of bug 1761855 (or visa-versa). I have the same problem on my system.

If I had a full example email, I could check if the structure and encoding are valid or not, and what happens when we try to process it.

I can try to provide an email that experiences the problem. Bug 1761855 has an example encrypted message body that could be used to create such an email.