If the short request works, but not the long request, and the long request is not TOO long (as in, longer than the model allows,) then the problem is with the configuration of your proxy, which is likely something on the inside of your VPN provider if you’re going through the VPN.
I would recommend trying the same request without a VPN (perhaps on a mobile hotspot) to see what result you get, and compare. Or, if that’s still not possible, at least try a totally different VPN provider/software.
This is almost certainly something in the chain between you and openAI, rather than something openAI can do anything about.