when you enable the filtering and try to browse, is there any error message from MWG or the ISA server? Is the browser "stuck" waiting for a response or just showing a blank page?
Without further data the configuration looks OK so far. Did you ensure that the ISA server can talk to MWG on port 1344? Maybe there is any kind of firewall in the line or the ISA server is blocking the connection? You could run a packet capture on MWG and check if packets come in.
I would have a look at this first to ensure connectiviy is given.