Okay so I know some people have received this error already. Personally it only happened to me once. It's the error where the Pre tells you to close some cards in order to free up memory. Usually it won't let you open any more cards until you close some (or all, in some cases). I found out that if you just go to card view (swipe up on gesture area or press center button) and wait a second or two, you should be able to now open an additional card.

This usually happens when an app is on high memory priority because it is loading/receiving information. Putting it into background mode (when in card view) should lower it's priority. Of course, after you open whichever app/card you wanted to, you can switch back to the memory-hog one and let it update/download/do whatever it needed to do.

Tell me if this works for you. Not sure if this was just a little trick that happened to work just for me so I would appreciate it if you could confirm/deny that it works. Thanks!