Excerpts from Fons Adriaensen's message of 2011-11-21 23:42:32 +0100:
I really don't know what login managers do. In the 'login' case it's
easy, login, startx, done. I think it's still worth a try. If that works
you could investigate what Xdm actually does.
No idea about runlevels either, sorry.
One further idea though: try enabling/disabling kms (kernel option
'nomodeset'), see whether the problem persists. The arch wiki says
nouveau needs kms, but it's not always right.
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