I hope you understand german. Do you have “Speichern oder öffnen von Anlagen, die Viren enthalten können, nicht zulassen”( I think it could be "Do not permit(allow) open and save Attachement that could contain Viruses"in english Versions of OE) enabled? You can find that option under “Option/Security”(Optionen/Sicherheit). You have to disable it, if you want to use these Attachements.