RSS feeds broken, possibly friendly URL-related

[lazy llama]

I noticed that I was getting 404 errors for requests to forum RSS feeds - <forum url>/index.rss
This had been working until recently, so may in my case be down to nginx location reconfig.
Using the non-friendly URL (with index.php?) gives the rss feed expected.

However, on checking the XF community forums I found that they are also broken, though in a different way.

Each RSS feed, e.g. https://xenforo.com/community/forums/bugs/index.rss
gives

The site is currently being upgraded...

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RSS feeds broken, possibly friendly URL-related

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