Maximum number of concurrent clients

I have a ZyXEL ZyAIR B-1000 V2 wireless access point that I got some years back.
We have bought new computers so we now have 3 wireless clients.
Two online clients’ work fine. When we turn on the third and try to access the Internet all Internet connections on all the PC’s fails and we have to reboot the ZyAIR before we can reconnect.
Our Router connected to an ADSL connection should not have any problems handling that amount of clients.
Is this a known error that can be corrected?