Yet another example of Microsoft's inability to provide a polished implementation of, well, anything.
To overcome the HTTP 500 Something Went Wrong frame you get when trying to access Exchange Admin Center > Mail Flow > Rules in Incognito Mode, Microsoft helpfully mention that this is a problem in a yellow info banner and to fix this enable third-party cookies. Yep, all of them. Hmm, no thanks.
To save you some time, here's the cookies you need to add to the allow list:
You'll still get the yellow info banner, but at least the Rules will now be displayed.