That message is a standard error message when the IP.Chat users limit is reached. The free version from IPB (Invision Power) standard edition which comes free with every IPB license is 5 users. In order to up the limit you are required to buy the IP.Chat license. They range from $10 (every 6 months, 20 users) upto $40 (every 6 months, 250 users).
Just thought i'd give my knowledge on the subject.