Security Update 2007-004 Special Report: Flash playback not working
Some users have problems playing back Flash content after the update.
MacFixIt reader Michael writes:
"Since installing the latest security update, Safari will no longer play YouTube video, although I have no such problem with Camino. Nor do I have an explanation."
You may want to try uninstalling Flash (using this uninstaller) then re-installing it (Intel or PowerPC). Another potential fix involves navigating to the /Library/Internet Plug-Ins directory then removing the following file
- Flash Player Enabler.plugin
For more fixes, see our tutorial "Flash content will not play in Web browsers, resolving."
Index:
- Active Directory login issues
- AirPort connectivity issues, fixes
- Cannot connect to local servers (Error -35)
- Files in Finder cannot be renamed
- Flash playback not working
- FTP security issue
- Login problems: users cannot login after update

Did not work for me. Unchecking and reinstalling Flash player did the trick.