Search working on Homepage – results in 404 on other pages/posts

I’m using a custom template – Paradigm – which I acquired through ThemeForest. I have gotten no reply from the author and I am unsure if this is theme related or not. I’ve disabled all plug-ins and re-enabled them to troubleshoot – no plug-ins to blame.

If I put in a search on the homepage of my site, I get results. When I go to another page or a post, and type in anything in the search box, I get a 404. I noticed that it’s because the /s? is being appended to the end of the page/post URL in lieu of being re-routed to the myurl.com/s? address.

Is there a quick and dirty way to fix this? I found similar posts, but they were very old and the solutions required editing pages that no longer exist in the current version of WP.

Many thanks,

S