I think I already found the cause of this. The probem seams to be that Openhab binding constructs a string from the bytes to pass this to the rules and it does that with the default charset:
sb.append(new String(readBuffer, 0, bytes));
My guess is that the default charset is UTF-8, so i changed this to ISO-8859-1:
sb.append(new String(readBuffer, 0, bytes, Charset.forName(“ISO-8859-1”)));
And now it seams to work perfectly! The question is if this should be the default charset or if this should be customizable.