Quicktime Player Problem

I have an iMac and have had no trouble playing videos and the like on Quicktime until the last few days. When I attempt to play a Youtube or other video, the Quicktime logo appears with a ? mark in the center of it. What's happening??

Did you run Disk Utility to repair permissions after the QT upgrade? That might help. According to Apple, occasionally the user permissions associated with files or applications become damaged, which can prevent a file or application from working as it should. See this article. So, you might want to try to run Disk Utility (Applications > Utilities > Disk Utility). Open the application, select the mounted volume for your hard drive and click "repair disk permissions."

If it doesn't help, open your QT preference (System Utility > QuickTime) and recheck all preferences. Pay close attention to the Advanced section in that window, and especially the MIME Settings there.