Pressing F5 does not refresh a stale webpage

The HTTP proxy's cache may have been set to operate in offline mode. This turns off the validation of cached objects and gives access to more cached information. It may cause stale objects to be sent to the client even when the client sends F5 refresh command. In this mode the only way to get a fresh copy of a cached website is to clear the proxy cache.
To disable this go to Proxy > HTTP > Cache Management and uncheck the option Enable offline mode.