I just got off the phone with the Wix support and they told me to post here because they officially don’t support utm parameters yet 
Anyway, here’s the problem: if you go to https://www.exaas.de/product-page/auswahl-fertigungsverfahren/?utm_source=Test you will get a “Product not found error”. Whereas if you delete the “/” before the question mark, the page works. I try to use the working url whenever I can, but some tools add it by themselves.
Is there anything I can do?
You will probably need to make your item page into a router page, which will allow you to check the url before the page is loaded. That way you could redirect to the correct page by changing the path:
if (path.join('/').includes('/?utm_source')) //replace it with correct path
Thank you @skmedia . I decided to create a dynamic page instead and use a custom database instead of the default products database from Wix.