..I wrote to WP Rocket and they answe that: (for your information) maybe you can help us more:
“Hi,
N… from WP Rocket here. Thanks for contacting the support ??
Sorry for the issue you are having!
I checked your site and was able to reproduce it.
I am not sure yet what might be causing it, but adding the shaveclick.ch/?coupon=profity15 string to the Never Cache URL wouldn’t solve this.
Question mark after slash means that it is a query string. WP Rocket doesn’t cache query strings by default, that is why it didn’t let you add the URL to the text area.
I checked further, and I can see that this query string redirects your visitors immediately to the default domain shaveclick.ch which is cached. I also checked the check-out page https://shaveclick.ch/warenkorb/, and it is not cached – which is a proper setup.
I suspect that when your visitors are following the link, a cookie for the discount should be set in their browser. It is possible to exclude pages from caching when a certain cookie is set by adding the cookie value to the Never Cache Cookie text area. But at the moment I cannot tell you what cookie it is. And I am not sure if this would solve the issue.”
Maybe these information will help you finding out why its causing the problem. ??