You may try changing the 8443 to diff port
Stop the 'Symantec Endpoint Protection Manager' service.
start all programs - symantec endpoint protecion manager.
select management server configuration wizard
2. On the 'Welcome to the Management Server Configuration Wizard' screen, select the option to 'Reconfigure the management server' and click on next
Change the port number specified in the 'Server Port' field to an alternate port that is available and not in use by any other application. The DEFAULT port is 8443
give it any port which is not blocked by ISA.
and click on next , dont make any changes on the subsiquent screens.
try now , it should be fine.