I think I found the real problem. It’s in fact not a problem of WordPress but a problem of the database. Just look at your posts with phpMyAdmin and manually set the browser’s character set to ISO. You will then notice that all special characters don’t consist of two (or more) characters but of four (or more)! So, they are double-encoded.
For example “st?ndig” becomes “st?¤ndig”, which becomes “st???¤ndig”.
? -> ?¤
? -> ??
¤ -> ?¤
