This bug is also related to Gmail IMAP.
it won't work correctly when downloading the body of the message.
Try downloading only the headers and check mails from the web.

Sometimes it goes thru and downloads the whole messages, many times it does not. Too erratic as behavior.

Anyway the should be a daemon running in background that's trying to download your e-mails, a bug is here that prevents the GUI to appear if this daemon is stuck into the download process somewhere.