Warning: Illegal String on WP Custom Fields Search

Hi,
I am new to wordpress and I am working on building a classifieds website using wordpress and ClassiPress. I have the newest version of WordPress and WP Custon Fields. I have used the WP Custom Fields Search but at the top of the page it shows:

Warning: Illegal string offset ‘preset’ in /home/content/p3pnexwpnas06_data03/50/2360850/html/wp-content/plugins/wp-custom-fields-search/wp-custom-fields-search.php on line 401

You can also view it at:
https://ottbclassifieds.com/advanced-search/

Does anyone know how to fix this? Also the search does not return any results.
Any help would be great!
Thanks in advanced!!!

https://www.ads-software.com/plugins/wp-custom-fields-search/