Hi,
Thank you for the details, it helps us a lot!
There seems to be an issue with either your template, plugin, firewall or the server. To test, I deactivated the plugin, then visited the search results page. The same issue appears, so it is not in relation with the plugin unfortunately.
The best way to get around these issues is by turning off all plugins one by one, to see if it starts working again. Also switching the template temporarily to see if it is not related to that.