This computer does not support video conferencing

I'm using an i-mac as administrator sharing it with one other user.

for the administrator user works everything perfectly even after the 10.5.4 update.
with both applications, ichat and photo booth.

But using the second account in the computer isight still working with photo booth, but not with ichat, here comes to the error communication:

This computer doesnot support video conferencing.

thanks for advice