Same issue here. Started about a week or 2 ago, give or take. and it's not all the categories.
I use RSSOwl and I have broken out about 12 individual feeds based on category, and there are 2 URL's are failing currently with an "Invalid or Malformed feed" which is really just a generic failure message in RSSOwl.
This one is very specific (cat 12)
https://www.gaytor.rent/rss.php?num=99999&cat=12&passkey=xxxxxxx
And this one is like a "catch all" for the feeds I didn't need to break out, so it could be any one of the categories that is the issue**:
https://www.gaytor.rent/rss.php?num=99999&cat=1,7,63,32,35,38,40,65&passkey=xxxxxxx
**If i remove category 35 from the above URL, it works fine.
So a least 2 categories seem to be broken (12 and 35), but not I don't pull feeds for all categories so there could be more.
(passkey has obviously been edited for this post)
As BigJohyn noted, plugging each into FF simply prompts to save the file rss.php, which if I do and edit, looks like XML as expected.I have not run it through an XML parser to verify it is valid XML, which might be a next step.
My guess based on my experience of what can go wrong in XML, a torrent in the feed(s) has character(s) that are problematic. (aka, improper un-escaped special characters, etc). I say that because I had a third feed that failed for just a few days in the beginning and then resolved itself, like whatever it was angry with had worked itself though the aging process.
Thank you
-Donny