One thing to check off the list: is it incorrectly detecting the browser? (UserAgent or other dubious detection method.)
Also, I would complain to mubi.com Support. That’s the kind of error message that I hate. Tell them to produce a decent, specific error message - so that you either can fix the problem or you know not to waste your time.
I would guess “DRM issue” too - which probably implies “don’t waste your time”.