I'm banging my head against the wall with this one.

I have set up PPTP server on a CentOS box. Unfortunately it's behind a NAT router.

Anyway, I can connect via internet no problem with a windows machine. However, when I try on a mikrotik box it fails. It says connection established but freezes.

I'm running other PPTP connections on the same mikrotik box, so it is related to my PPTP server setup.

Any ideas? Is this to do with GRE? I have PPTP pass through enabled on the router.