Mail encryption not working on iOS and iPadOS 16

I had iPadOS 15 installed on my iPad, self-signed profile/cerificate imported, mail encryption worked fine. Upgraded to iPadOS 16 and suddenly, I cannot encrypt outgoing messages any more. Whenever I try to do that, I get the following message:

Unable to encrypt

You can't send encrypted messages because an encryption identity for the address .... could not be found. Go to the Advanced settings for this account to choose an encryption identity.

Od course, I do have the identity selected in the refrerred settings section. And strangely enough, encrypted incoming mails are being decrypted and displayed correctly, which means that the mail client does have access to the private key of my profile.

Exactly the same is happening with my iPhones. Everything works fine on iOS 15 and I see the same problems on my other iPhone with iOS 16.