I made a quite funny experience, in my case the problem turned out to be as follows...
It was just a problem with cache/cookies.
I saw in the inspector of my browser that a request for the rendered version of the readme was sent,
but returned with the answer, that it is already rendered. (Which it of course was not...)
I deleted all cookies, cache and session data for the site in my inspector and after refreshing it worked!
(Guess that not the browser cache, but a cookie was responsible, because I had the same prob in different browsers...)
I don't see any "rendered" cookie. Also, clearing cookies and cache doesn't have any effect for me at all (except that I have to log in again).
What I see, is that a GET request is sent to the server to render the README.md. The answer (which arrives after more than 2 seconds) contains a structure containing the "raw" and "html" fields, but both contain the raw MD, ie. the html field does not contain any html at all. So it seems to be a server side issue.
I have the "New source browser experience" still enabled, BTW. When I disable it, the README is rendered correctly.