Cause:
Reply / forward try to decrypt the original message *again*, but only if `KMime::Message::isEncrypted()` returns true (which it does not in case of mails mangled by outlook).

The whole class of problems could be avoided by not re-parsing the message. That needs some extra API in mimetreeparser, though, to either allow retrieving the parsed message from MessageViewer classes, or allow those to work with an existing ObjectTreeParser instance.