Is it possible that your page is just using a different layout than it had been before? I would check that it is still set to whichever layout had the header you are looking for. This screenshot seems to be of a sidebar layout, I would look for something containing the full layout with header and all.
Maybe you are not using the right Layout:
Check also if the Modules is called Atlas_Core like you can on the image above.
Hey Miguel and Luke,
first of all, thanks for your help.
But I found out what the problem was. I started the migration all over again with this guide: https://docs.mendix.com/refguide/moving-from-atlas-2-to-3/
Then I saw that I messed something up with my custom-variables file. I had some imports that didn’t make sense.
After adjusting eveything the header came back.