GTM Server does not get Data

I used this how to: to Setup Facebook Conversion API (step 1 to 9 - also configured the needed Constants Server URL, Preview Header, Facebook Pixel ID, Access Token and Test ID).

Now in my Web Container I see the events in preview mode like page_view, view_item, add_to_cart. But I don’t see any requests in my Container here GTM Server nor in my GTM Server. Whats missing here?

Little update: now I see that the requests in my container from app.gtm-server is not 0. But my GTM Server does not show anything when looking into preview, nor do I see Server Events in Facebook

I found out the problem was that Preview Mode variable from Server Container changed. Is this only changed when the container is re-published? After updating the variable in Web Container it worked.

Ok the Preview Mode Variable changes all the time. How should that work long term? This solution just breaks when Preview Mode Variable changed. Please help

Preview header changes when you publish the server container. It can also be changed several times a day. I suggest checking/updating the preview header every time you start working inside the server container and every time you publish the server container.

Ok thank you. But it is only needed when I want to track down the events within Preview of the Server Container?

When everything worked one-time, the events will be forwarded without problems to Facebook Conversion API even if they are not visible in the Preview of the Server Container because the Preview Header was updated and is not set correctly in the Web Container and the settings of Tagging Server from

Yes, the Preview header is required only to see Data Tag requests inside the preview mode of the server container.

Ok but also when I am trying the Facebook Event Test Tool it does not show me the Server events when the Preview Mode of the server container does not work. Is that correct behaviour or should rather work the Facebook Event Test Tool all the time?

Here you can see how it looks when the Preview Mode does not work due to wrong Preview Header, it shows only Browser Events:
Screenshot 2021-06-04 at 10.27.14

FB test ID is added as a variable. This variable works only when the preview mode is enabled. You can add FB test ID as a constant and check if you see events inside the FB testing tool.
Screenshot 2021-06-07 at 09.36.36