Hello,
Here are a few things to consider:
1.The feed URL should be in the format, here are the correct URLs to use for creating an RD Web feed:
For On-Premises RDS: (https://<rdweb-dns-name>.domain/RDWeb/Feed/webfeed.aspx, such as https://rdweb.contoso.com/RDWeb/Feed/webfeed.aspx)
If you're using Azure Virtual Desktop (classic): https://rdweb.wvd.microsoft.com/api/feeddiscovery/webfeeddiscovery.aspx
If you're using Azure Virtual Desktop: https://rdweb.wvd.microsoft.com/api/arm/feeddiscovery
see: Set up email discovery to subscribe to your RDS feed | Microsoft Learn
2.Ensure that the SSL certificates are correctly configured and trusted on the client machine, as certificate issues can prevent access to the feed.
3.Check that the necessary ports (such as 443 for HTTPS) are open on your firewall and that there are no network restrictions blocking access to the feed.
If the Answer is helpful, please click Accept Answer and upvote it.