I’m using the mailchimp extension for contact form 7… that’s what was causing the error. I fixed this issue by doing the following in /wp-content/plugins/contact-form-7-mailchimp-extension/lib/functions.php
Change line 223:
if ( $special = apply_filters( 'wpcf7_special_mail_tags', '', $matches[1] ) )
to
if ( $special = apply_filters( 'wpcf7_special_mail_tags', '', $matches[1], '' ) )
notice the extra parameter in the apply_filters function call.
How was the issue fixed? Having the same issue and there is currently no plugin update available.
Thanks!
