Hello Anton,
there is a mantis entry for this bug and a fix (thanks to studer & raimann for informing us):
http://www.ilias.de/mantis/view.php?id=17837
https://github.com/ILIAS-eLearning/ILIAS/commit/0f570f6db32e8d778285df8c4b696b2415406897
We applied this fix to our 5.1.2 installation and are now able to set the sorting with SOAP again.
Best regards
Claudia