I think the problem is in your WinCC configuration.
At first you have to set the rack/slot comination to rack=0 and slot=2 (not 0/3 as you have configured). A S7-300 has always this combination.
The next thing is the 'logical device name' in system parameter settings. Maybe you have special PC hardware, but I set up usually something like "TCP/IP -> ......", ending with the name of the LAN interface I want to use (don't use something with 'Auto' in it).