For anyone interested, it was the SEOyoast plugin that caused the problem.
I turned it off and on, than the page appeared again in the admin.
After that I rest the settings for the page and it now works as expected and accepts its template.
For anyone interested, it was the SEOyoast plugin that caused the problem.
I turned it off and on, than the page appeared again in the admin.
After that I rest the settings for the page and it now works as expected and accepts its template.