If you add <appInstallFromUnknownSrc>2</appInstallFromUnknownSrc>under vendorConfig you will be able to install Apps from Unknown sources. Hope this helps.

Your post was very useful.Thank you very much. I stumbled upon the same problem while trying to set the time on a 9971 connected to Fritz!Box....and after looking here I found a solution on a German page. Simply search for "sip-date.pl cisco-8961-995...