As people have already said, you can only enter 5 instances per hour. Once you get the message you simple have to wait a little while.

I get this message myself from time-to-time, most recently was on the weekend when I was farming Magisters TCE up to the first boss. I was trying to hit Exalted. Running to the first boss rince and repeat. With only 5 pulls including the boss I could be in and out within 8-9 minutes.