I tried on my Mobile (Windows Phone with Edge, yeah I know) and search works. I tried with Firefox on a Win10 box and it works, trying with Edge on same box and it does not work. Spoofed the user agent string to Firefox, chrome, etc and it does not work.
When I say it 'does not work' I mean it returns results but if you click on a result you get the error message. So a search result link looks like:
https://retropie.org.uk/forum/post/16612
If you click on it the URL in the current window will change to that link and then it gets 'redirected' to:
https://retropie.org.uk/forum/topic/2874/converting-tronkyfran-s-theme-to-720p/1
Something in this redirection process is throwing off Edge. Funny thing is when the error is displayed the 'redirected' URL us currently displayed so refreshing the page works.
Why can't we just have the actual URL returned by the Search tool???