I tried proxying through a server in Germany (in a big, well known hosting provider) and I got the same message, both with ssh -D and pptpd. So they're either

I tried proxying through a server in Germany (in a big, well known hosting provider) and I got the same message, both with ssh -D and pptpd. So they're either reading signatures left on the packets by various VPN / proxy technologies (fragmentation and other obscure metrics), or they're blocking the IP ranges of major hosting providers, or both.