The following info may not help you get the resolution you need but it is the best explanation as to why Knoppix is unable to do 1280x786.

Unless I am mistaken, Ubuntu uses the more recent X.org while Knoppix still uses the older XFree86. This difference in X subsystems could result in Knoppix not supporting "non-standard" resolutions or configurations. Knoppix as a derivative follows the path of Debian development quite closely. It is likely that a future version of Knoppix will use X.org since Debian itself is transitioning to X.org.