It appears that you could very well have two notification applets. Could you try to reproduce the issue under a new user to verify if the duplicate applets could be the cause of the issue?

This i can't test i'm afraid for i created a new user and in it's session pidgin opened being empty ...
it's a pity i do not have access to all the information to set it as in my regular user session.
Is there a way to copy / paste some data so that it is set as in my regular session ?

I would suggest looking for application configuration and data directories, likely stored in ~/.config and ~/.local for Pidgin.

hello,
in .config and .local i do not see anything linked with pidgin, is there something ?
i found /home/user/.purple, which i copied to the new user but in its session pidgin still is opened empty of settings.

I would suggest searching for "gconf" in this case - as Pidgin is a GNOME application it is probably using the primary GNOME configuration system for it's data.

i have some directories in .gconf , but no pidgin in /home/user.gconf/apps

If what I recall is correct, GConf is rather similar to the Windows registry - it uses one big file for all applications. You might need to copy the ~/.gconf directory completely over.

hi,
if i copy my usual user .gconf dir to the newly created user /home the result is the same : pidgin is still opened empty.
If i click on pidgin in the new user session it does not trigger kde notification pannel.
I guess i would have to recreate all the settings of my actual session to be able to compare both situations, but it's not really interesting for it is not easy and certainly would not suffice to have an exact duplicate,

Okay, rather unfortunate that GNOME applications such as Pidgin make this quite difficult. Instead, could you try temporarily moving your Plasma Desktop configuration out of the way, so it starts with fresh defaults?

The configuration files are named "plasma-desktoprc", "plasma-desktop-appletsrc" and "plasmarc", and should be located at ~/.kde4/share/config/

hello,
do you mean saving them and renaming them in my usual user session ?
if i do this : kde settings are brand new, and pidgin does not trigger kde notif pannel anymore

Okay, assuming your default Plasma setup includes the notifications applet, then that indicates the problem is somewhere in your old Plasma Desktop configuration. Given that configuration issues with Plasma tend to be fairly difficult to resolve, I would suggest setting it back up again from scratch - as determining what specific elements in the Plasma configuration caused this could be fairly time consuming.

Exactly, i let it as it is now and set it again in kde, which is easier that in Pidgin (in my case).
Thanks for your help