Have you installed/updated Glib2 (2.14.4)? or gtk+2.12.2?
I have a similar problem, I even made a Thread
about it. I created it a a while back.
I am compiling my own Gnome 2.20.2 in my Slackware Machine, so i have to upgrade some libraries in order to build it (Glib, gtk, pango).
Opera works fine, and so does konqueror, but as soon i upgraded the gtk/Glib libraries, im not able to watch Flash animations in neither one of those Browsers. There is no problem with Firefox.
Ive been searching the net for a while, trying to find out which library is the one to "blame", and if there is a patch or any known way to solve this issue.