Problems with encodings

I have some problems with my encoding on my WP 1.5 Blog. It’s a very strange think. On my Wp 1.22 Blog everything works fine. No Problems there. All German Umlauts are displayed correct there.
But I want to update my Blog to the new WP 1.5. I started a second Blog directly with WP 1.5 for testing. And if I want to post some text with special characters, it doesn’t work. It works only if i declare them with html code like & ouml ; -> ö
But that’s no solution.
Strange is, that my sidebar content shows the characters in the right way. For example my amazon media manager or the blogroll.
And strange is, that everythin works correct on my local LAMP System. On my PC all Characters are displayed correct.

When I change the encoding in WP from utf-8 to iso-8859-15 then the new posts are displayed correct. But I don’t want to change it, cause the future belongs to utf-8 and all my older posts will be corrupted.

I have no Idea where the Problem is and how to solve it.