Sorry, that's my fault. PPPoE/PPTP & NAT stop working - just figured out this. For now the easiest way to restore the normal operation mode is to execute
iptables -I FORWARD -p tcp --tcp-flags SYN,RST SYN -j TCPMSS --clamp-mss-to-pmtu
after the connection was established to the pppoe/pptp server.
I'm now trying to determine the exact reason for that...