When moving site from one domain to another everything worked except from the WP RSS Multi Importer plugin.
To solve the problem I have deleted the plugin, and re-installed it. But with the same error message: "You have one feed and it's not valid. This is likely a problem with the source of the RSS feed. Contact our support forum for help."
Category option are set to: FEED
RSS Feeds is set to that category
The feed: https://www.webcruiter.no/webservices/webcruiterculture.asmx/RSSFeed?company_id=1234782315&CultureId=DA
Any help will be appreciated!