Two weeks ago I decided to upgrade to kde 4.2. Everything went fine.. well almost… I managed to login to new kde once and later made some mess (btw. new kde looks very nice). I didn’t use any upgrade guide and for some reason I was unable to log in through KDM (but loging using xdm was still working).
Only suspicious errors in logs were
[config/dbus] couldn’t register object path
Most straightforward option for me seemed to be downgrading again to kde 3.5.9, but after that problem still occured and is described here and here.
Before you try above solutions first create new user (with no files in his HOME directory) and try to log in onto his account. If this works then the cause of your problem is some old/wrong config files in $HOME.
Unfortunately I don’t know which file was causing problem in my case but for sure it was not any file from .kde* directory (and previously only that files I had checked)
…almost three weeks of using xdm… (additionaly “turn off” option also dissapeard so in order to shutdown system I had to “poweroff” from root (saving session in kde obviously does not work in that case)).
Having that fixed I’m going to try on kde 4.2 soon again.