When replying to a message, it wants to use encryption sometimes. I enabled encryption but not for all messages. So it can not send the message as the recipient is not listed for pgp and no key found. But I did not tell to encrypt it. So I can not send the message and must cancel it and write the answer again.
It is not always the case. But I did not see a pattern yet, when it tries to encrypt it. Now I had to disable encryption in total to answer all emails.