Downgrading from nvidia-glx-new to nvidia-glx does not remove the file /lib/linux-restricted-modules/.nvidia_new_installed. This causes the wrong nvidia kernel module to be loaded (it loads the nvidia-glx-new module, instead of the nvidia-glx one), and causes an API mismatch.
The fix is indeed to remove the /lib/linux-restricted-modules/.nvidia_new_installed file, this ought to be done when removing the nvidia-glx-new package.
I can also confirm this bug.
Downgrading from nvidia-glx-new to nvidia-glx does not remove the file /lib/linux- restricted- modules/ .nvidia_ new_installed. This causes the wrong nvidia kernel module to be loaded (it loads the nvidia-glx-new module, instead of the nvidia-glx one), and causes an API mismatch.
The fix is indeed to remove the /lib/linux- restricted- modules/ .nvidia_ new_installed file, this ought to be done when removing the nvidia-glx-new package.