I'm seeing the same error as was reported here https://www.mojoportal.com/Forums/Thread.aspx?pageid=5&t=12986~1 but on a different blog.
2019-01-22 08:52:19,131 ERROR 82.69.43.209 - en-GB - /FeedManager/FeedEdit.aspx?mid=583&pageid=273 - mojoPortal.Web.FeedUI.FeedCache - There was a problem trying to read the feed for url https://mysite.co.uk/Blog/RSS.aspx?p=200~180~-1&g=20ae54ae-f501-48eb-a1f9-c4b700a8bcf1. Ignoring.
System.Xml.XmlException: Data at the root level is invalid. Line 1, position 1.
...
The feed URL works fine accessed directly in a browser, and loads XML that is superficially identical to the XML from another feed that loads fine in the Feed Manager. I've tried removing the feed, clearing cache, adding again (to the Feed Manager) - no change.
Any ideas?