> If the Google AdSense code was the problem,
Yes, GA was problem. First problem.
> The header dissapeared and stopped working at the exact moment I installed and deactivated WPGlobus
You are wrong again.
I have installed new, clean DB.
1. WordPress v.4.9.4
2. Theme Astrid v.1.15
3. No WPGlobus, no any others plugins.
Step 1:
I have uploaded image for Header Image in customize.
I have added texts for Header text.
Then I clicked Publush button in customize.
It works. I see correct image on front.
Step 2:
I have uploaded one more image for Header Image in customize.
Then I clicked Publush button in customize.
It works. I see correct new image on front.
Step 3:
I selected image by default (it is part of the theme).
Then I clicked Publush button in customize.
Oops, it is not working.
I see the change in customize, but on front I see previous image.
I reload customize page and my change was lost.
So, theme has some issues with Header Image in customizer.