The very kind lucero at the WordPress support forum posted a fix for the problem with norwegian characters:

echo utf8_encode(html_entity_decode($title));

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!