Please re-install the latest version of OI Safe manually from here:
(without first un-installing)
The problem occurs after a system update that introduces new permissions to access the SD card. In the previous system these permissions were not necessary, and therefore not granted. The applications need to be reinstalled in order to be granted permissions to access the SD card.
From Market you can only install a newer version, but manually you can install the same version on top of the existing version. This should do the trick, and you should not loose the data under normal circumstances. Just don't uninstall - simply install on top of the existing installation.
In any case, please write the most important passwords on a piece of paper before you proceed, just to be on the safe side.
The German error message is:
"Sicherungskopie erstellen fehlgeschlagen: /sdcard/oisafe.xml"