I'm using following:
xoops: 2.3.3
xampp
windows xp sp3
browser: firefox 3.5.3
UTF-8 encoding for all languages

When some pages use redirect_header and displays some text as redirecting message, on some pages are displayed as ISO-8859-1 encoding, some pages displayed as UTF-8 encoding.

For example logout message displayed always as ISO-8859-1 encoding. But my other languages coded as UTF-8 and my texts are displayed as garbage :)

Any idea?

Thanks

Check your database, mainfile.php and global.php, all have the UTF-8 encoding.
Clear the caches.
Update the system module, maby the others too.
(Some modules may have separate redirect pages with a fixed encoding.)