Hello @Manfred Winter , and welcome to Azure!
I created a lab and updated my template like in your example. From the Lab Services website I:
1) Clicked on Export to Shared Image Gallery
2) Created a name and clicked on Export
After waiting up to an hour, I went back to the portal. At this point, I am able to create a new lab using my Shared Image Gallery image but I can confirm that my image was exported successfully:
3) Navigated to my Lab Services resource
4) Went to Policies > Shared Image Gallery
5) Clicked on my Shared Image Gallery
From there, I could see that my image was successfully created. If I click on the image, I can see that it is given a default version of 1.0.0
Now when I go back to the Lab Services website, if I create a new lab I'm given the option to use the image I created for my Shared Image Gallery:
It sounds like you are running into problems after exporting your image. What happens when you look for your exported image in your Shared Image Gallery?