Feed does not validate

Hi, website is https://www.lactivist.net
The feed used to work fine, but it looks as though the url has changed recently, it is now https://www.lactivist.net/feed/ instead of
https://www.lactivist.net/?feed=rss2

On networked Blogs I get
“Feed not valid

We tried to process your feed but it seems to be formatted incorrectly. This indicates an error in the blog software. Please contact your blog host provider or your Web admin for assistance. Once you fix the feed on your side, please try again.

This is the error we encountered, please pass it along to your Web admin:
<unknown>:133:570: not well-formed (invalid token)”

I asked on Networked Blogs and they said it is nothing to do with their side.

I checked it on Feed Validator and it says
“Sorry
This feed does not validate.”
and got a whole heap of errors I just don’t know where to begin fixing – I’m really wary of messing with the PHP in case I break the site and I don’t even know which page it’s talking about that the errors are on.
https://feedvalidator.org/check.cgi?url=http%3A%2F%2Fwww.lactivist.net%2Ffeed%2F

Any help for a complete idiot would be much appreciated, it worked and now it doesn’t and I don’t understand what I have done to break it.