Server dropped connection unexpectedly with certain WiFi

OS: Yosemite Type: Macbook Air 11

I have 2 WiFi's (A and B). Whenever I connect to A, my Mac drops the connection to a certain websites and often loses connection to the WiFi. But when I connect to B, everything's fine. No disconnection and no "server dropped connection unexpectedly". Perfect.

Why is this happening and what can I do to solve the problems when connecting to A?