While trying out Jitsi, I encountered that annoying problem.
Everywhere a video is supposed to be displayed, (that is, in the test
frame from the option window, and in call windows), I get a gray frame
instead (see screenshot ). My camera starts recording correctly
however, and during video calls my interlocutor can see my video (using
Google Mail's video chat plugin, actually), but I cannot see his.
I thought that could be related to the window manager I use since Java
is known to have problems with non-reparenting WMs, but I reproduced the
problem with GNOME which is a more classical one, so it seems unrelated.
PS: Sorry for having opened an issue  in the BTS without discussing
it first, I guess I simply forgot to read the instructions…