This is probably caused for some incompatibilities between router firmware and linux kernel.
Please provide more information as kernel logs for example, also you can check this from pptp's site.
http://pptpclient.sourceforge.net/howto-diagnosis.phtml
and from ubuntu wiki.
https://help.ubuntu.com/community/VPNClient#PPTP
This is probably caused for some incompatibilities between router firmware and linux kernel.
Please provide more information as kernel logs for example, also you can check this from pptp's site.
http:// pptpclient. sourceforge. net/howto- diagnosis. phtml
and from ubuntu wiki.
https:/ /help.ubuntu. com/community/ VPNClient# PPTP