Hi,
Thanks for the details it helps a lot!
There seems to be a mod_rewrite or URL rewrite rule applied for the search results page, and it tries to redirect the search results base from the default ?s=phrase to /search/phrase/ URL, but it does not seem to work properly.
If I try to visit the search results page simply by going to https://magellanmotorcycletours.co.uk/?s=test
..then I get the same error. For other phrases (for example: https://magellanmotorcycletours.co.uk/?s=phrase ) redirects to the modified URL but there is still a 404 error reported.
Make sure to check this custom URL rewrite/mod_rewrite .htaccess rule, it does not seem to work okay, it is very likely the cause of the issues.