@danix: You are right! It does indeed fail for an entirely different reason. It does not have anything to do with the Nvidia drivers but with grub/plymouth/Ubuntu.
I found out that selecting recovery mode in grub but then resuming the normal boot, there aren't any issues at all anymore. So what are the differences between normal boot and recovery boot except for no "quiet splash" but "recovery in the grub command line?
The answer is, some weird Ubuntu stuff in /etc/grub.d/10_linux, "Native boot entries", especially the line "gfxmode \$linux_gfx_mode". What it does: makes the boot process beautiful with plymouth, which works great as long as you use the default ubuntu kernel from the repos. When you have issues with that kernel and want to try a different one you run into serious problems like I described in the post above. Thank you Canonical!
But really thanks to nvidia for their new driver, its awesome to have dkms support inculuded.