The normal mail app correctly shows a list of mails in my mailbox including a preview of the message body. When I tap any of them, the mail opens and correctly shows the mail header (sender, subject, date etc.) but the message body is blank.

When I look at the same mailbox using Gmail webpage, I see the identical mails with message body intact so I know that it's not a problem with the mails or the mailbox itself.

I'm using not-broken iOS 4.1 on an originally Apple-unlocked iPhone 4.
I'm using Google's official setup for Google Apps on iPhone.
I've tried purging the mail/contacts/calendar settings and following Google's setup from scratch. The problem is still there.

How can I fix it so the message body is displayed?