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I have recently come accross this problem, and I am posting the solution I just found. The problem is the following: when trying to watch any online flash animation with Iceweasel on my Debian Lenny (e.g. in YouTube), instead of the video, I got a grey window, with the following black letters on it:
This SWF file is known to trigger bugs in the swfdec decoder. Playback is cancelled.
This bug can be followed in this August 2007 thread at donarmstrong.com. I am amazed why it didn’t affect me sooner than this week, but oh well…
The steps that fixed the issue for me where to install two packages. I am not 100% sure the first one was needed, though. I first installed the package flashplayer-mozilla, which uninstalled flashplugin-nonfree as a side effect. This step alone fixed nothing.
The second step was to actually read the donarmstrong.com bug report, and see that it says that “This is fixed in swfdec-0.4.2 [...]“. Great! I did a aptitude search swf, and found out that Lenny has libswfdec-0.5-4, and a swfdec-mozilla that depends on it. I installed the latter, which obviously also installed the former, plus it removed swf-player and libswfdec0.3.
After that, I opened Iceweasel again, and now flash works!