Broken Boot-Splash and Textmode after Upgrade to Natty 11.04

Yesterday, I've upgraded from Ubuntu 10.10 to 11.04. The upgrade procedure worked flawlessly, but while rebooting, there was a white screen with black vertical lines instead of a boot splash. After bootup, the graphical login screen and GNOME resp. Unity showed up correctly. But when I try to switch to a console (e.g. [Alt-F2]), I only see the strange white screen with those black stripes again, so it seems to be an issue with text mode only.

I have installed the proprietary driver for my Nvidia Geforce 7200. I've also tried to change the boot options (only ro and nosplash), but the result is still the same.

Does anybody know a solution or a workaround for this issue? I don't need any bootsplash, but I would be happy to have a working text mode bootup and console.

Thanks
Heiner