Thanks a lot. Setting the environment variable solved my problem. But I think the NVidia driver 302.xx doesn't solve the problem but made it appear on my computer. I'm already using version 302.17, but some months ago dispwin didn't show this behaviour and everything worked fine with the NVidia driver without setting ARGYLL_IGNORE_XRANDR1_2.According to the google groups posting, 304.30 beta is the one that fixes gamma loading.
Ah ok, after reading the Phoronix article, I thought the xrandr support of 302.xx should have solved the problems, but maybe NVidia introduced a bug that effects my configuration. Let's see what 304.xx brings...