RSS feed need validating

I have no idea what happened, but suddenly my RSS feeds needs validating. Always worked like charm. But the combination with mailchimp is broken, all of a sudden.

See here what I received from validating service.
https://validator.w3.org/feed/check.cgi?url=http%3A%2F%2Fwww.amsterdamtrader.com%2Ffeed

It gives
XML parsing error: <unknown>:55:92: not well-formed (invalid token)

and
Use of unknown namespace: com-wordpress:feed-additions:1

Suppose there’s a plugin causing trouble? How to solve it, quickly?