Can you try disabling Shields for the site by visiting it, clicking the lion icon in the address bar and toggling them “off” and see if this makes any difference?
It is likely some protection we offer that is (I suspect Fingerprinting protection) causing the error. Either that or the UA string is not showing up as Chrome and that is what is throwing the “unsupported browser” message because the site should function the same way it does in Chrome.