Standard RSS integration on Matrix.org returning error for all RSS links tried

[FrederikSchack]

I tested the standard RSS integration at Matrix.org through the Element desktop app and get the same error for all the RSS links I try:
"Unexpected token '<', "<!DOCTYPE "... is not valid JSON"

Amongst others I've tried these RSS links, that I can normally read with InoReader (https://www.inoreader.com/):
https://www.theorganicprepper.com/feed/
https://www.collective-evolution.com/feed/
https://reaction.life/category/world/feed/

All of the feeds are valid, only some recommendations for the two first links:
https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fwww.theorganicprepper.com%2Ffeed%2F
https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Fwww.collective-evolution.com%2Ffeed%2F
https://validator.w3.org/feed/check.cgi?url=https%3A%2F%2Freaction.life%2Fcategory%2Fworld%2Ffeed%2F

So, I suspect that the RSS integration is not working properly.