Finally fixed my friend's Gameguard error 3xx problem.
I had them use OpenDNS.
https://www.opendns.com/home-interne...-ip-addresses/

Symptoms of the problem:
- When running PSO2, Gameguard downloads files, then gives a 3xx error.
- PSO2.exe closes itself, does not run.
- There is no PSO2.exe process left in memory.
- Same problem on all PCs using the same ISP.
- Same problem on different routers but same ISP.
- Using different ISP from a different location works.
- Merely flushing the DNS cache does not work.

I figured it was either the router or an ISP problem.
Seems the problem was their ISP's DNS cache.