The very kind lucero at the WordPress support forum posted a fix for the problem with norwegian characters:
instead of echo $title;
in wp-includes/feed-functions.php line 12
will fix this problem.
Fantastic! It solved the problem with norwegian characters breaking the feed. If I now could get some more feed readers to accept my feeds everything would be fine! Right now Mozilla Thunderbird is the only reader that accept the feeds… I have no idea why. This was not a problem with my MovableType blogs.
I need to find a solution!