Within the configuration settings for the RSS Viewer Web Part, the checkbox is selected for either "Enable Asynchronous Update" or "Enable Asynchronous Automatic Refresh". It is expected that the RSS Viewer Web Part will update the web part asynchronously. However, the web part is not updated and instead the web part may display the following error: "An unexpected error occurred while rendering the web part."
This is expected behavior with the RSS Viewer Web Part. The web part was not designed to support this asynchronous functionality yet the option was not hidden in the user interface.
The RSS Viewer Web Part will not update asynchronously. However, it has been improved in that there is now the ability to enable data view caching for a configurable duration and data from the RSS feed can be refreshed at a configurable interval within the web part configuration settings. The web part will not update independent of the page, but will update when the page is refreshed in the browser.
RSS Viewer Web Part; An unexpected error occurred while rendering the web part