At the end there is no usuable call from the RocketChat client. But I can copy the meet.jit.si URL and open it in a browser.
Originally I had an error message telling me that Windows doesn't know what to do with jitsi-meet:// links, but that doesn't show up anymore for some reason. Maybe because of me messing with the registry to solve the issue, but I've actually removed the registry key I had created before.