The only way I could imagine how TCP/IP settings could cause a web server to produce a 403 error is that you come in with a suspicious IP address. Which IP address did you assign to your Amiga? IP addresses in your home network should be in the form 192.168.x.y where x is the same for all computers at your home and the same as your router and y is different for each computer (i.e. unique). If you use something like 1.2.3.4, it could be interpreted as fraud.
