Running Win8.1 RTM with patched-to-current copies of Chrome, IE, and Firefox. Attempting to start any in private/incognito mode by right-clicking and choosing the given context menu item will fail silently. No error, nothing left in the Event log - just no resulting window.
CommandLine iexplorer -private and firefox -private (haven't found Chrome start-up switch yet) will work.
I'm the sole user of the workstation - running as Admin - only working with Desktop apps (not metro) - no reason to suspect infection. puzzled.
ADDENDUM: Turns out the that most of the choices offered at the 'RightClick From Taskbar' for all three browser apps will do nothing. For example - the only context menu item for chrome that works is the one that literally opens Chrome (and unpin from taskbar). None of the others will execute anything. Doesn't matter if Chrome is already open or not - nothing happens. Firefox = same misbehavior.
IE's 'Frequent' items will respond (opens the given URL) but nothing in 'Tasks'.