I was experiencing problems with all flash content (tried Firefox/Chromium/Opera, Ubuntu/Fedora and several Flash releases to no avail). While trying to playback any flash-movie, no video/sound playback occurred, and right-click only produced a "Movie not loaded" warning.
I accidentally narrowed-down the problem to a user-specific tweak I made in my '/etc/security/limits.conf':
my_user_name soft stack number_of_kbytes
The interesting part is that, whenever
number_of_kbytes > 2097151 = 2^21,
this problem occurs, otherwise things are fine flash-wise.
Does anyone know why this may happen and where does this "magic number" come from? Also, is there a way to increase stack-size limits per-program (I have a C++ program which needs this sometimes).