<p>'c3 85' is the UTF-8 encoding of U+C5, yes. I wondered if the username uses a combining diacritical, i.e. U+030A, but tried 'A' followed by that, <a href="https://www.openstreetmap.org/user/A%CC%8Andrew" rel="nofollow">https://www.openstreetmap.org/user/Åndrew</a>, and it gives a similar result. It could well be Andrew does something fancy with his Unicode username and it's being canonicalised, or otherwise altered, before ending up in the URL that's faulty.</p>

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