Cannot broadcast to clients from a thin client

update: I'm trying with a little class (9 students) and broadcasting from a thin client (with LDAP teacher) works on all clients.
I'll further investigate.

edit:
I got the same error with little class too.
If i click "share desktop to.." the error pops up.
then, if i click "share desktop to" another time /without/ clicking "stop sharing", it works on all thin clients.

if I make some tries starting/stopping desktop sharing, some clients became unresponsive (not only from epoptes: I need to swith 'em off long-pressing power button)

* Did you say that when you try the same stuff directly on the server, there's no problem at all?
* How much RAM do your clients have?
* Do you get client crashes (more rarely) when not using VNC? It sounds like it might be network or graphics card related...
* Are all your clients and your switch ports gigabit? Especially the thin client where the epoptes runs, and the server, and those 2 switch ports, need to be gigabit.

Trying from server, I didn't experienced errors, nor when not using vnc. ...but the installation is pretty new, so I couldn't be sure.
Clients have 512MB RAM
unfortunately, there's a 100Mbps switch in this lab, but I have same issues in another lab with 2GB clients and gigabit lan.
Moreover, if it would be a hardwprovlemblem, it shouldn't work simply clicking on sharing icon a second time..

As said, with second click desktop broadcasting works great on 20 clients.. with the first, sometimes didn't work nor with a single client.

I wondered if the problem could be linked to the installation I made of epoptes-client on LTSP's chroot (to make thin clients available before login). But as I have the problem before login too, I don't think it could be relevant

Try this:

sudo gedit /usr/share/epoptes/client-functions
change line 257 to:
        sleep 2.$(($(hexdump -e \"%d\" -n 2 /dev/urandom) % 50 + 50))

(the only change there was 0 to 2)
Save, exit

sudo service epoptes restart

Reboot clients and see if it helps.

Thank you.
I remotely applied your suggestion a couple of days ago but, as I'm not at school, I'm waiting a confirmation from the lab.
As soon as I'll receive it, I'll update this thread.

Hi.
they said that we don't have the error message anymore, but with the first click not every client gets the shared screen.
With the second, all of them gets connected.

PS: I was able to connect the first lab to a gigabit switch too.

What are your server specs? Maybe it's a bit slow and it can't receive many simultaneous client connections if they all come within 0.5 sec?

Let's try to increase that to 1 sec instead:
         sleep 2.$(($(hexdump -e \"%d\" -n 2 /dev/urandom) % 100))

...I also wonder if it's related to DNS issues...

I'm really sorry, but I'm not able to get a feedback from lab on your latest suggestion.
I'm hoping to have one in the next few days. :-/

Sure, no problem, but please also try this:
$ sudo gedit /usr/share/pyshared/epoptes/ui/gui.py
And change line 299 to:
            self.vncserver = subprocess.Popen(['x11vnc', '-noshm', '-nopw', '-threads',

In a couple of weeks a new epoptes version will be published that (among other things) will enable threads support in x11vnc. This will make broadcasting faster, but it might also help in your case.