After setup pptp vpn tunnel on Debian VPS ( pptpd ), you can access most of sites without errors. But if you find you can’t open some sites like: www.marca.com, then this is more like a tcp mss size error. You should ssh into your VPS erver, and run the command:
iptables -I FORWARD -p tcp --tcp-flags SYN,ACK,RST SYN -i ppp+ -j TCPMSS --set-mss 1356