Comment 4 for bug 156320

While update-manager shouldn't fail in this manner, the extraordinary thing here is that $XAUTHORITY is set and points at a non-existent file. update-manager will be unable to establish any further X connections under those circumstances, so aside from the ugliness of the error, the real bug is in whatever has set up $XAUTHORITY to point to a nonexistent file (or has removed the file that $XAUTHORITY was pointing to).

Hence it's necessary to know how update-manager was invoked in order to trace down that bug there.