maybe an apache problem with non-ascii characters?

[onr]

I am not sure if this is a php (i.e. coppermine) or general apache problem so I apologise if it doesn'T belong here but maybe someone here has come across it since I spent the last 2 days trying to find a solution with google and didn't succeed.
Ok I am running a very hacked version of Coppermine in which I have applied SEO friendly urls.

One of the lines in my .htaccess file for instance goes like this:

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#search
RewriteRule ^search_(.*)$ thumbnails.php?album=search&type=full&search=$1&%{QUERY_STRING} [L]


So a URL like search_hamburg will expand to thumbnails.php?album=search&type=full&search=Hamburg

So far so good, but if the search string contains non-ascii characters like äöü or é è etc it does not work anymore.

If I try search_Lübeck for instance, the browser location changes to search_L%FCbeck and I get a 403 error. So I assume the Rewrite rule is not found or something funny is interpreted, because if I enter it manually in the location bar as:
thumbnails.php?album=search&type=full&search=Lübeck the ü again changes to %FC but it works!

The strangest thing is that this problem only occurs on my developing server on my local machine and not on my ISP's apache server. We both use the same apache (2.2) and php (5.2.3) versions so I really don't know what the problem could be. Maybe an entry to the httpd.conf file or the php.ini ?!? Anyone who has had this problem as well?
Many thanks in advance.

P.S.: I have tried all the usual suspects like

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AddDefaultCharset ISO-8859-1 in httpd.conf or

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default_charset = "iso-8859-1" in php.ini