WooCommerce has default functionality that prevents caching on the checkout and my account pages. There is only one page from Diller that should be excluded manually from caching:
/innmelding-kundeklubb/
The procedure will vary since there is a wide range of plugins that handle caching, but in most cases there is a dedicated menu where you can easily add the page above. This prevents the sign-up form from being cached, and the problems that can arise as a result.