It appears the problem is because mailcrypt expects a single string
of comma separated id, but it was given a list -- because
message-recipients is a list.

It is interesting to note that using the above patch, vanilla pgp
encryption works, as in I get queried for a list of recipients and the
corresponding public keys are fetched etc.  Perhaps, this is because
mml2015-mailcrypt-encrypt is invoked more than once on a message, but
mml1991-mailcrypt-encrypt is invoked only once.

-- 
"Have you ever fought an IDEA, Picard?"
      -- Gowron, Star Trek TNG.