I have done a search however could not come up with any explanation for this problem.
I have a BB 8700c and use it with my company's BES and Lotus Notes 6.0. When receiving long emails and trying to "get more" after scrolling to the bottom of the email I get the error message "can't get more". When I exit the message back into the inbox there's a red X next to the message.
This also occurs when replying to meeting requests.
Could be an issue with the communication with your BES. If you're still getting e-mail, try a hard reset (remove the battery for 30 secs), and see if that helps you out.