I retried to login today after 2 days have passed and the prompt for password change no longer occurs. This means, the system is somehow confused by a recent password change throughout different sites.
The RSSImport-plugin uses WP’s rss-function and works pretty well w/ WP-feeds but as soon as a feed has its own XSL it produces errors.
Better use PHP’s simplexml_load_file in this case
$sxml = simplexml_load_file($feedurl)