Yoast breadcrumbs isn’t html5 compliant.
So when the validator sees that the Theme IS html5 it will assume all the elements are html5. This causes the breadcrumbs to fail the check.
Could you post your url?
http://wordpress.org/support/topic/breadcrumbs-do-not-validate-as-html5
http://bloke.org/uncategorized/making-wordpress-seo-html-5-compatible/