"Encryption has problems"

I had the signing and encryption working fine between two different computers. Today my 2nd computer died (SSD drive failure :frowning: ) 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”

When I click on the “Encryption” blue letters, this is what I see:

(the serial number does match the 2nd certificate, emclient just can’t seem to find it!)

But I do see them both in the certificate store within Personal / Certificates and I don’t see any properties looking inconsistent between them:

Even within emclient, it can see both:

Interestingly, doing the opposite, originating a signed email from the 1st email and encrypting the reply from the 2nd email works ok.

Any ideas?