New Post: Error "Failed to get page template by id"

burningice - Thanks for your feedback that it couldn't be connected with the Template store 'Composite.Data.Types.IXmlPageTemplate_Published.xml'

I have overcome the problem but haven't a clue why it happened.

I changed the IPage xml store for the specific page with a another template id in the site.
that allowed it to open in the dashboard, where the problem template then was listed - I reassigned to target template and saved in the production site - et viola, it works.

A pain in the 'rear' as I do not know where/how the problem occurred. Four hours wasted, but at least I know more about Templates storage ;)