RTF Export fails to create proper .rtf file

I was trying to export a number of pages, all of which contained German "Umlaute" (Ä, Ö, Ü, ä, ö, ü) and "ß". Additionally, some of the pages included tables.

Upon trying to load the .rtf files in MS Office 2003 SP3 or LibreOffice 3.3 RC1 I got error notices. Wordpad (as of Windows XP), however, could open the RTFs. Problem was, that all the Umlaute and "ß" and the character right after them were replaced by "?".

Example:
Änderung --> ?derung

LibreOffice is connected as an internally managed OOo Server.