Hmmm, can you try running gdmgreeter instead of gdmlogin?
I tried it (forgot to mention, sorry) and it looped and bailed out as well.
I just captured a debug enable=true syslog and it looks similar.
It seems like the SIGUSR2 handling between processess is somewhat broken.
It is more likely
that either a new version of the gtk/gnome libs is hitting a bug in the
greeter or the other way around. So perhaps seeing if the graphical greeter
works might be an indication of such problems.
I admit that I should not run all that latest stuff.
By the way I am not complaining, (I am a big boy ;-)), all
I want to to is rapport and help.