Non-xml feed (552) error

I’m getting a “Non-xml (552)” error for this feed:
https://newsblur.com/site/8598348/perspectives-on-science
https://direct.mit.edu/rss/site_1000061/1000033.xml

Looks like the RSS feed doesn’t validate:

https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fdirect.mit.edu%2Frss%2Fsite_1000061%2F1000033.xml

Ah okay. It looks like another journal from the same press has similar configuration problems. I found a webmaster contact, so I’ll try bugging them.