Magento 2 – “The value specified in the URL Key field would generate a URL that already exists.” even though the value does not exist in url_rewrite

I get “The value specified in the URL Key field would generate a URL that already exists.” but I checked the table “url_rewrite” and there is no entry with that key.

The URL is “verbrauchscalculator“. If I call the URL in the browser, then a page opens.

There is also no category where the name is “verbrauchscalculator”.

What can I do?