RSS feed no longer validated

I have been using the RSS feed for my website (www.materials-risk.com) to populate an email to my subscribers using Mailchimp. A few weeks ago the emails suddenly stopped as the RSS feed is no longer validated. I can still see my updated RSS feed in Feedly however.

When I put the feed link through feed validator it comes up with the following errors.

line 21, column 125: Undefined channel element: div [help]
… f=”http://pubsubhubbub.superfeedr.com”/><div style=’text-align:center;ma …
^
line 21, column 239: XML parsing error: <unknown>:21:239: not well-formed (invalid token) [help]
… p:8px;margin-bottom:8px;’><script async src=”//pagead2.googlesyndication …

I have tried disabling all my plugins and then gradually reactivating them. I have also activated the Twenty Fourteen theme to see if it is theme specific. I have also tried using the Fix My Feed RSS repair plugin but with no result.

Appreciate any help. Thanks

Peter