Hi !
I made a completely clean installation with
OpenWrt SNAPSHOT r5986-3f7908d / LuCI Master (git-18.031.70145-518ac52)
- Install the packages
opkg update
opkg install kmod-nf-nathelper-extra
- Add a line to the file /etc/sysctl.d/local.conf
net.netfilter.nf_conntrack_helper = 1
and reboot router
PPTP Passthrough works !
Many thanks to @cezary for the short and clean answer.