Which rights are needed to add widgets to the desktop? I installed xprivacy on my rooted device and blocked all privileges to the "Google Search"-app. Now i cannot add any widgets any more.

Which rights do I have to allow and how could I find out such questions my self?
That's usually the INSTALL_SHORTCUT (resp. UNINSTALL_SHORTCUT) permission – which might be present multiple times, as it's declared by the resp. launcher. To find out yourself, just follow the links I've given :)

EDIT: As stated in their FAQ, Xprivacy is no "permission manager", but a "privacy manager", and thus only cares for privacy-related permissions. Installing shortcuts is one of those permissions it does not care for (explicitly stated), so this is rather an unintended side-effect to be dealt with an update (fix) from the Xprivacy dev(s).