I had this problem occasionally with Zen last year, messenger, https sites, and other seemingly at random. However not everyone on Zen got it. Turns out one of their servers was semi knackered, but they have several.
The easy work around was to jump servers. Do this by disconnecting your modem.. wait ten minutes then reconnect. If you are lucky then the lottery of servers means you might be on the functional one. Worth a try in a pinch.