Can't read RSS Feed from Joomla 4 using different RSS-Classes

Hello,

I built a litte RSS-Feed-Reader for my companies intranet site built on joomla cms, to inform the users of new articles showing a windows taskbar balloon message using visual studio .NET application.

Everything was working perfect for years, but now I need to upgrade to joomla 4.

I created a test intranet page and upgraded it to joomla 4. Everything works fine, but now I run into a problem I can't solve for about 4 weeks, driving me insane.

My application was designed to use the System.Xml.Linq class to read the rss feed.

string feedUrl = "https://some rss feedurl";

XDocument rssfeedxml;
XNamespace namespaceName = "http://www.w3.org/2005/Atom";
rssfeedxml = XDocument.Load(feedUrl);

If I run the application I get an Exception :

System.Xml.XmlException: Es sind mehrere Stammelemente vorhanden. Zeile 2, Position 2.
bei System.Xml.XmlTextReaderImpl.Throw(Exception e)
bei System.Xml.XmlTextReaderImpl.ParseDocumentContent()
bei System.Xml.Linq.XContainer.ReadContentFrom(XmlReader r)
bei System.Xml.Linq.XContainer.ReadContentFrom(XmlReader r, LoadOptions o)
bei System.Xml.Linq.XDocument.Load(XmlReader reader, LoadOptions options)
bei System.Xml.Linq.XDocument.Load(String uri, LoadOptions options)

So I tried to switch to using System.ServiceModel.Syndication class

XmlReaderSettings settings = new XmlReaderSettings();

settings.ConformanceLevel = ConformanceLevel.Fragment;

var reader = XmlReader.Create(feedUrl,settings);
var feed = SyndicationFeed.Load(reader);

If I run this code, I get also an Exception :

System.Xml.XmlException: Das Element mit dem Namen 'br' und dem Namespace '' ist kein zulässiges Feedformat.
bei System.ServiceModel.Syndication.SyndicationFeed.Load[TSyndicationFeed](XmlReader reader)

If I switchback the feedurl to joomla 3 everything works fine again on both classes.

If I use the joomla 4 rss-link in FeedReader 3 the feed is loaded.

If I download the xml file using the rss-feed link and do a xml-analysis in notepad++ no problems were found.

That drives me crazy. What is the problem on this?

best regards

Bl0ckS1z3