This problem occurred in a custom environment where the web player window is made to open in a different window by means of a 'onclick' event. E.g.

onclick="window.open('https://testdrive.nomachine.com:4443', 'newTestdrive_window', 'width=1024,height=576'); return false;" "="">Testdrive</a>

This raises the following exception in Google Chrome 89:
Uncaught DOMException: Blocked a frame with origin "https://testdrive.nomachine.com:4443" from accessing a cross-origin frame.

while Firefox v. 78 reports:
Uncaught DOMException: Permission denied to access property "z15..." on cross-origin object webplayer-shared.js:30