Hi Dan,
thanks for getting back to me with this issue. Yes, it’s been happening with a blank project, when I add pages. So I’m sure it wouldn’t have helped a lot if I uploaded an empty project here. 
But the good news: the problem is solved! 
The key to the solution was your hint with changing the theme. I had set „Foundation“ as the default theme for months (or years?) now. But when I changed the default theme to „Alpha“, then quit RW and restarted it, the issue was gone. Then I selected Foundation as the default theme again, quit RW and restartet it. And… from there on everything’s working just fine when adding new pages, no error message anymore. Pheeeww…, what a relief! Once again thanks a lot for your help!
Best, Matthias