Hi @Westerberg, Eline
Thanks for visiting our forum. Microsoft Teams tag is mainly focused on the general issue of Microsoft Teams application troubleshooting. Based on your description, this problem is related to RSS APP, please consider posting in the corresponding Microsoft Community for better help. Thanks for your understanding.
Based on my knowledge, the RSS Viewer Web Part cannot consume RSS feeds from another SharePoint site without some additional configuration by a server admin, including enabling Kerberos authentication on the SharePoint farm where your site is hosted. It cannot consume any RSS feeds from SharePoint in Microsoft 365 sites.
For more details about adding RSS feeds from external sites to your site, you could refer to:
Hope this helps! Moreover, if the above is not your scenario or I have misunderstood anything, please correct me and provide us with detailed information about the issue including the screenshot so that we can better understand to further assist you.
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