It seems the messages aren't downloaded onto the N900.
Go to
/home/user/.modest/cache/mail/imap/YOUR_EMAIL_ADDRESS__imap.gmail.com_993/folders/INBOX/
and see if any of the unreadable messages are the in there.

Just for the record my last readable message wa 16:12 BST on the 11th and my first unreadable message was 23:25 on the 11th