Not working with Safari: "Domains, protocols and ports must match."

For some reason, opening files in Collabora does not work in Safari:

Unsafe JavaScript attempt to access frame with URL https://cloud.****.org/s/******** from frame with URL https://office.****.org/loleaflet/ffc419a/loleaflet.html?WOPISrc=https%3A%2F%2Fcloud.****.org%2Fapps%2Frichdocuments%2Fwopi%2Ffiles%****&title=Anmeldung%20zum%20Workshop%20Zusammenleben%20im%20*****.xlsx&lang=de&closebutton=1&revisionhistory=1. Domains, protocols and ports must match.

No response code found: HTTP/1.1 400

Since Collabora must run on a different domain, what am I supposed to do about it? It works fine with other browsers. What can be done to tell Safari that this is just fine and safe?