External XMPP Client - Private Message

I got Pidgin (same problem with gajim) setup as external XMPP client for the codingame chat.

It works great with the chat channels, but …

My Problem is when someone sends me a pm a new window open and i see his messages, but all my answers are going to /dev/null …

the window is titled for example 1464932@chat.codingame.com/CGWebchat-f249b73e49

An idea for a possible reason
As far as i remember XMPP correctly the part after the / is the resource the user is currently using.
Lets say a user has currently 2 resources Webchat1 and Webchat2
I can write to 1464932@chat.codingame.com/Webchat1 and the resource Webchat2 won’t see the msg
If i write to 1464932@chat.codingame.com the priority of the resources determines which resource will receive the msg

I guess CG assigns a resource per Webchat instance that could mean that the resource i got the msg from doesn’t exist anymore
so if i respond the msg gets lost …

an example that i just have seen i gajim
[13:25:32] ‎1257742‎: test test
‎[13:25:50] ‎Tiramon‎: can you read this
‎[13:29:39] ‎1257742 is now offline

and he definitly is still in the chat …