It's not about pptpd, since the clients and the server are both windowses, nevertheless it's still pptp traffic (i.e. tcp 1723, proto 50 <esp> and proto 51 <ah>)
The kernel built from sources. I was thinking that, since one connection passes through and the nat and conntrack helpers are loaded for pptp and gre, it should work.
These protocols are not explicitly nat-ed, but the main nat line is
$c -t nat -A POSTROUTING -s $intlan -o $extif -j SNAT --to-source $extip
Furthermore, those protocols/ports are allowed in the FORWARD section.