For those that want to know the details, I was using this shortcode:
[ics_calendar title=false description=false view=week legendstyle=none compact=true url=”https://www.volleyscores.be/calendar/club/10713?|?https://www.vks-limburg.be/team161.ics?|?https://www.vks-limburg.be/team177.ics”]
And apparently, after my first URL I did not put the ‘.ics’ extension, which was needed for it to work. In my defence, it always worked like this, maybe the source I got the URL from did some backend change, I don’t know. But they deliver the URL without the ‘.ics’.