got stuck with the same problem.
i removed (dumb me :) ) the lightdm-webkit2greeter and updated the system. after that i could not reinstall it ( also dumb i know :D )
so i need my notebook tonight so i tried around a bit.
1 workaround that worked (for me except!):
after removing lightdm-webkit2-greeter i updated the system, then i tried installing the lightdm-webkit2-greeter from the AUR (ver 184.108.40.206)
that didn’t worked also.
so edit the PKGBUILD file as follows:
change the following line
depends=('lightdm' 'webkit2gtk>=2.10' 'webkit2gtk<2.12' 'gtk3>=3.18' 'gtk3<3.20')
depends=('lightdm' 'webkit2gtk>=2.10' 'webkit2gtk<2.13' 'gtk3>=3.18' 'gtk3<3.21')
after that i could update and start my system with a working X-Server ;)