Hi,
The URL "domain.de/wasser" is like that because the alias of the menu item used for that URL is "wasser".
I suppose that your "wasser" category is selected as the "main category" of the "wasser" menu item.
However, you might have several menu items for that same category. So the module doesn't necessarily know which menu item you want to use for a category.
Also, suppose that you have a menu item "shop", and this displays a listing of categories. And for the "wasser" category, the URL is "domain.de/shop/wasser" where "shop" is the alias of the menu, and "wasser" is the alias of the category. The module can't match the category with the "main category" of the menu item, and it's not evident that the parent category of that category is selected in the "main category" of the menu item.
So, by default, the module will use a generic menu item to generate the URL of the category. That way, the page of the category will display with that generic menu item regardless of how you construct your menu items tree structure.
Now, we've made a mechanism so that you can tell the system which URL you want to use for each category. For that, you want to enter the URL you want in the "canonical URL" of each category. Then, activate the "force canonical URLs on listings" setting of the HikaShop configuration. And now, your module will switch to the canonical URL instead of trying to generate the URL itself and you'll get the URLs you want.
You actually have the same mechanism for products.
And having a canonical URL for your categories and products is not only nice to get consistent URLs for your users, it's also good for the SEO of your website.