xorg - Unity does not start despite having worked for several months

My computer has been able to run Unity with no problems since I installed 11.04 a few months ago. The unity_support_test script have previously always given a firm yes to the question if Unity is supported. My graphics card is a NVIDIA GeForce GT 330M and I have installed the proprietary driver version 270.41.06.

However, Unity did not start today when I started my computer and logged in, instead I got the "Ubuntu classic" interface. I have restarted a few times and been extra careful to choose "Ubuntu" as session and not "Ubuntu classic" or any other non-Unity alternative, but the result is the same every time, Ubuntu classic and no Unity.

There were some updates yesterday, but I did not recognize anything interface related.

I have read some other questions and answers here, but I did not find anything useful. The only recommendation that I have followed was to install the CompizConfig Settings Manager and to check if the Ubuntu Unity Plugin was activated, but the plugin was already active as it should be.

As a final check before posting this question, I runned the unity_support_test script and to my surprise the answer was not "yes" but instead the following:

mghg@vaio-ubuntu:~$ /usr/lib/nux/unity_support_test -p
X Error of failed request:  BadWindow (invalid Window parameter)
  Major opcode of failed request:  138 (NV-GLX)
  Minor opcode of failed request:  4 ()

  Resource id in failed request:  0x1ff
  Serial number of failed request:  31
  Current serial number in output stream:  31

Please help me get back to Unity!

Reinstalling the drivers fixs the issue.