504 Gateway timeout

Hi, Recently we purchased a Fortigate60B with FortiOS 3.x...The FortiOS was upgraded to 4.0.0.092....We had configured it to work in transparent mode with our exi ADSL router as gateway ...no VDOMs were defined...only ' web' and ' strict' protection profiles were enabled for incoming and outgoing traffic.. It was working fine until last monday when we started to get the error ' 504 gateway timeout' in our browser when we tried to open the web-gui with the ip address of the FG unit... earlier we were unable to solve the similar problem and had done the factory-reset from command line interface...but it reoccurred... Plz help us to solve the problem...plz post ur suggestions...