Thank you for the log.
Looks like a server bug to me -- the server is not returning any content when my app asks for a specific part within the message.
I realize this is not helping you much, and you'll probably come back with "but mail app <X> works"....
Well, ok. IMAP has different ways (commands) for a mail client to interact with the server.
K9 uses a more simple way to talk with the server. If that works for you, by all means...