Empty email body

We have a rather peculiar intermittent problem with confirmation emails. The buyer receives the email, but it is completely empty, apart from attached images (our logo and the ticket QR codes) which would normally appear as part of the message body. The copy of the email sent to the seller is fine.

Has anyone else experienced this glitch? As I said, it is only intermittent – perhaps about one in twenty bookings.

Thanks for any insights!
Graham