PPTP Windows client blocked by my RB750GL firewall ?

I can’t get a customer’s VPN running. He needs to run PPTP to a VPN provider. It would appear that the RB750GL is blocking him even though I have trivial firewall rules that are just for dropping inbound connections from the internet.

His VPN was working behind a toy (consumer type) router.

How do I isolate what I am doing wrong? Do I have to do something with GRE (which I know nothing about)?

It’s the same for me.
I don’t get a VPN over a CCR1016-Router working.
I’m sure, that it have worked with earlier Versions. Now the router use Version 6.19 and the VPN are not get established.

I use the packed sniffer to check the connection but there I can’t see a reason for this. All pakets are send correctly.