Thank you for the information, it has helped me find the bug.
It seems that when I set the interface language to German, I can no longer add new PPTP instances. I have registered the bug so it should be fixed in future FW versions.
Meanwhile, as a temporary solution, I suggest you set the language to English, add the PPTP configuration and set it back to German.
Thanks for registering the question and sorry for the bug.
Good luck,
DM