Permalinks problems

I’m only guessing but
are you using a category for the post?

you should have a separate post category, else it will look for the category in shop and unless you have a product category identical it will not find it.

this post https://dns.rockybeachstudio.de/november-2017-regal-von-katja-jobs/ states posted in Kundenfotos and the link is looking for Kundenfotos in shop?

You may find the below helpful:
https://docs.woocommerce.com/document/removing-product-product-category-or-shop-from-the-urls/

Yes, “kundenfotos” is the category of the post. If I remove the category path, which currently is “shop”, the post category and post paths are correct. But as I wrote above, as soon as I do this, the product categories all show the “page not found” error. Only if I remove all paths, both post and product category pages will work – but of course only with the stupid standard paths.

that’s odd, I was expecting it to show under post categories:

https://dns.rockybeachstudio.de/category/Kundenfotos

that’s where it appears when i remove all paths.

I think the link below should help:

https://docs.woocommerce.com/document/permalinks/

The note at the bottom seems to cover your scenario.

it covers only one aspect of what I’d like to achieve, which won’t work if that note is correct. but it doesn’t cover the current settings, where product base is set to default.

it doesn’t say anything about why it’s mixing up post categories and product categories and how to use them both.