In order to use the WordPress plugin with your ClickFunnels pages, it is best to leave your ClickFunnels pages on your default account subdomain (e.g. yourname.clickfunnels.com).
You should not add your WordPress site domain (www.wordpresssite.com) to your ClickFunnels account as a custom domain in your account settings.
You may add a custom subdomain of your WordPress site (customsubdomain.wordpresssite.com) for your ClickFunnels pages, but your ClickFunnels pages and your WordPress site should never share the same domain.
If you have already set your ClickFunnels pages' custom domain to the same domain as your WordPress site, you will need to change the ClickFunnels page domain back to your default account subdomain (e.g. yourname.clickfunnels.com).
Current Plugin Version
Always check to be sure you are using the most current ClickFunnels plugin version. This can be verified on the Plugins Page of your WordPress Dashboard.
Further ClickFunnels WordPress Plugin Assistance
If you are experiencing an error that does not fall under under any of these categories, and you have already deactivated all of your plugins and re-enabled them one at a time, please submit a support ticket at https://support.clickfunnels.com/support/tickets/new