Win8 Pro with automatic updates did not install KB2819372 (providing 11.6.602.171) correctly.
Result was that some flash-source (e.g. youtube) would play normally (metro and desktop!), other sources would not start (facebook, macromedia and adobe test sites that show the version, e.g. http://helpx.adobe.com/flash-player/kb/find-version-flash-player.html).
Also, the HP Solution Center (used to scan on HP devices) would not start, as it did not detect the IE plugin.
Error message was all alike: no Flash plugin installed, guiding me to the adobe download area (which does not work for Win8/IE10, as the plugin is provided by Win8), or leaving the flash area greyed out / empty.
After KB2819372 was manually removed and reinstalled via windows update, IE10 behaviour returned back to normal.
As I did not find any working solution for my problem, I thought I could share my experience.
Thank you for posting! I'll let our Win 8 team know and will keep this in mind if anyone else encounters this problem.