problem solved:

just go to ~/.purple and delete "prefs.xml".

a new prefs.xml will be created on next pidgin startup.

it seems that old/false preferences are responsible for this error.

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popups on maemo? i don't know...

the only thing i can imagine:
using dbus to show the messages

(the theme-colored banners which appear when something important happens - i hope you know what i mean xD)