cannot log in with GNOME after upgrading gnome-shell from 3.18.3 to 3.20.3

[Alexey Muranov <alexey muranov gmail com>]

Hello,

i have encountered a strange problem after upgrading NixOS from 16.03 to
16.09, which upgraded gnome-shell from 3.18.3 to 3.20.3: i cannot log in
with GNOME anymore.

I can still log in from console or with other graphical desktop
managers.

What is strange is that i have a second test account, and i still can
log into that account with GNOME.

What could be a cause for this behavior?  Which differences between the
two accounts should i look for to try to fix this?

Thanks,

Alexey.