Hi
if you have Joomla advance (Finder) indexing your site and have set up ES to work only for reg users and you have more langauges like english + others you can search for example a specific event in english version and get redirected to register/login to see the result. If you change to another language your get redirected to a 404 error message instead of get to the ES login/register page.
This must be a bug..