I’ve already setup the environment correctly base on the Technet guide.
The farm topology is 1 WFE, 1APP (which runs the Search Service Instance), 1 SQL and 1 Fast Search server.
This error happen after few months of system commission.
Whenever hitting a keyword via the Search Box, this error “Unable to connect the search services” kept throwing.
After checking the server system time, i noticed that there is 5++ mins difference between APP Server and Fast Search Server. This time difference will cause the web service call to fail!!
Hence, please ensure that your Search Service Instance server and the FAST Search are in synch with the time. I’ve heard that there is this NTP for servers to synch up the time. You may explore it in detail yourself =)