Works really well but…

I’ve tried this plugin on a development site, and it works beautifully.
I then tried it on an existing site which uses the same theme, and got the WSOD, apart from the single word “Forbidden”

I disabled all other plugins, but still got “Forbidden”

Any idea what might cause this? Or how would I fix it?

Thanks

Ben

https://www.ads-software.com/plugins/daves-wordpress-live-search/