MAPI Proxy: Decryption failed with error: required public key not found

PGP Desktop fails to decrypt encrypted messages. The PGP Log error message is:

MAPI Proxy: Decryption failed with error: required public key not found

I have reimported the relevant public key, signed it, activated it, set my Trust to Implicit, cached my passphrase, downloaded the relevant public key from pgp.keyserver.com and re-imported it. None of these things has fixed the problem, and I consistently get the same error message every time PGP Desktop tries to decode a message.