Thanks for the info. I was not able to generate a certificate on the Apple developer site since I was not paying for the membership. But since I posted this message I finally gave in and paid for the membership. Now provisioning works for me without a problem.
Also ARKit seems to be somewhat problematic with 4.17 and XCode 9. But I finally had success with the newly released 4.18 preview.
The ARKit Setup is also much simpler in the new version. Unfortunately I do not have a solution for 4.17.