UPDATE: I now found out why FUSA only displayed as a 1px vertical bar: the "desktop-switch" tool somehow set desktop/gnome/lockdown/disable_user_switching to TRUE which caused FUSA to this behavior. Since it is also used to set the current Pidgin state and to Logoff, I guess this is not the desired behavior...
UPDATE: I now found out why FUSA only displayed as a 1px vertical bar: the "desktop-switch" tool somehow set desktop/ gnome/lockdown/ disable_ user_switching to TRUE which caused FUSA to this behavior. Since it is also used to set the current Pidgin state and to Logoff, I guess this is not the desired behavior...