When receiving an encrypted email, the email contains an attachment in clear text.
Remove the disclaimer from your Microsoft Exchange server for the user and send the email again.
Please refer to the PGP Universal 2.9.1 Release Notes, "Adding text below a block of ciphertext in a message may cause PGP Universal Mail Policy to have errors when processing the message".
It might also help to set the Inbound mail policy for incoming mails to always decrypt "This rule is always executed" or to always decrypt for a certain domain instead of the default configuration "If any of the following are true: Any part of the message is encrypted to any key".
Microsoft Exchange Server using a Disclaimer. This applies to S/MIME and PGP encryption.