I tried to fix it by modifying .htaccess:
#RewriteRule .* index.php [L] RewriteRule .* index.php [R=301,L]
But this results in a "broken" redirect like:
Further I changed
System -> Configuration -> Web -> Url Options
and set "Auto-redirect to Base URL" to 301 (before 302). Cleared ALL caches in backend and var/cache - The online checker still show the same - 302.
Any ideas how to fix the issue? Thanks!