postbox / mailbox folder from IMAP email account not updated on iOS 8

IMAP account exists on OS X 10.10.2 and iOS 8. That account has several folders. When looking at the folders on iOS they do update fine and then display "Updated Just Now". But for one folder I see "Updated on 09/02/15 which indeed might have been the last time I visited the folder.

The odd thing is when I pull to refresh or just after entering that folder, mail on iOS says "Checking for Mail..." and here comes the trouble: then it goes back to "Updated on 09/02/15" and "No Mail" in the middle of the screen.

On OS X there's quite a few mails in that folder, also in the webmail client of the account.

What's stunning here is, that the other folders update just fine.