hi all. I'm hit by this issue too (mail not found in server yadda yadda).
one thing to note is that I'm still successful at moving emails that can't be opened to the trash in gmail via imap.
am using thumb2 since last 2-4 months & this just happened to me today.
hadn't updated to the latest CSSU yet.
I started having this problem last monday. It hapends with every email. but only with gmail I have two other acounts other than gmail and they work just fine.
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
Switching to Nokia Messaging gave me access to my mails once again. Some folks have noted that they were OK on Nokia Messaging and so I tried it and indeed it is one work-around.