Since a while Google sitemaps reports this error for a bunch of sites, but not for all. All sites are set to the www version, have the same site map coding and there are no redirects used at all.
Saw, that many people are having this issue, but could not find a solution ...
URLs not followed
When we tested a sample of URLs from your Sitemap, we found that some URLs redirect to other locations. We recommend that your Sitemap contain URLs that point to the final destination (the redirect target) instead of redirecting to another URL.