Quick reply...
I have just created the plugin, it loads and works.
The problem is: after Reboot, the applications went mad. File Manager replaced Nokia N900 with <unknown>, Settings put all buttons into Extras section, Quick Launch has lost its memory (forgotten previous settings and cannot remember new settings), X Terminal has lost "Tab" and "Esc" buttons, "Internal error: Application 'Photos' closed" notification appears, virtual keyboard cannot be shown, etc.
I'm now checking part of device's memory by connecting it in Mass Storage Mode and asking Windows to check the drive for errors.
What would you advise?
MBarcode and all its plugins work even in this chaos;-)

EDIT: "The permission of the setuid helper is not correct."
What is this?
Said by osso-product-info and gconftool

EDIT: N900 can live through anything!
I reinstalled DBus and DBus-x11 (downloaded .deb s from maemo.org), and everything is correct! The settings returned!
Now I only need to recharge it: dozen of reboots drained the battery somewhat.