[Users] GPG Encryption failing

[Bob Williams]

I am using Claws-Mail 3.17.8 running on openSUSE Leap 15.2 (Linux) and
KDE/Plasma desktop.

I haven't used GPG for a while, but now wish to encrypt messages to a
particular correspondent. We have both generated and exchanged new key
pairs. However, I cannot send encrypted messages to the other party,
getting the following Error message:

Could not queue message for sending:

Encryption failed: Unknown error

Signing the same message works OK. I have checked that both the signing
and encrypting sub-keys are present in my keyring and are in date.

I am not sure if this is a problem with the CM GPG plugin or gpg2 on my
system, so I thought I'd ask here first.

Any suggestions on how to track this down?

Bob
-- 
Bob Williams
   System:  Linux 5.3.18-lp152.50-default
   Desktop: KDE Frameworks: 5.71.0, Qt: 5.12.7 and Plasma: 5.18.5
   https://useplaintext.email/