This is a known bug in jquery validator plugin. However, this has been fixed now in this pull request, and should be released soon.

Fix 1:
If you cannot wait for a release, the fix is to put this code at the start of delegate() method. The solution has been made by @svrx.

// Set form expando on contenteditable
if ( !this.form && this.hasAttribute( "contenteditable" ) ) {
    this.form = $( this ).closest( "form" )[ 0 ];
}
As you can see the fix is simply to set the form to the parent element.

Fix 2:
As @chrisAtomix said in his comments, don't use the latest jquery validator plugin. Instead use a lower version where contenteditable feature has not been added. He is using v1.14.0.