Invisible icons in Virtue theme

I have a very odd problem with icons in the Virtue theme. I have the domain https://www.opmeeractueel.nl and the subdomain https://news.opmeeractueel.nl which I use as a kind if testing ground for changes I want to carry out on my main domain before I implement them there. On both the main domain and the subdomain I use the latest version of the Virtue theme downloaded from Kadence and they both have the some settings. I installed icons in my main menu and I used the Icon menu. In the subdomain they show up fine, but in the main domain they are invisible even on the backside. If I click “select icon” in the icon menu I get the list, but not the icons in front of it. However, the most curious thing is that I can see in my browser that the icons of the main menu of the main domain are really there, but invisible. If I hold my cursor not only over but a little above e.g. HOME on my Mac it changes from the arrowhead into the pointing index finger as proof there is a link there. Any ideas anyone?