Using the GTMS with only measurement protocol is a hell of a setup. Not that it is complicated, it is just not well documented.
– You need to deploy the initial container (yes, I should have know that from the normal GTM) – but I installed 3 server tag managers until testing that
– When you are testing the preview you need to add a token to your header data X-Gtm-Server-Preview – you can find the token by clicking on the three dots in the top right
Now it’s working…
I think that will become a blog post.