You can check the subnets of all VPNs (servers and clients). Maybe same subnets are being used.
I don't use PPTP but, both OpenVPN server and client is active on my router and all working fine.
Thanks a lot for the advice.
I tested the openvpn server with the orginal asus firmware of my 68u before, but it didn't worked with an active client.
where I can check which subnets are used, in the advanced settings of the pptp server only the IP adress 192.168.10.X is listed. The router normally works in the 192.168.1.X net.
My local IP of my vpn client is identical with the pubic ip according to the vpn status site....185.x.x.x