I had the signing and encryption working fine between two different computers. Today my 2nd computer died (SSD drive failure ) so I tried set up the 2nd email account additionally on my 1st computer - and installed the certificate there which I had exported and backed up luckily.
But now, when I send an email from the 2nd email, signed. And reply to it from the 1st email, encrypted. When the 2nd email receives the reply, it says “Encryption has problems”
Interestingly, doing the opposite, originating a signed email from the 1st email and encrypting the reply from the 2nd email works ok.