Hi @ps224327,
This comes from the fact that at the moment the SSL is not set up correctly on your site.
At the moment, the link without SSL also works and the form has that link as action: http://ehealth.topzorggroenveld.nu/sign-in/
While using a plugin like: https://ww.wp.xz.cn/plugins/really-simple-ssl/, you can fix this issue.
Best regards,
This fixed my problem. Thanks!