I have recently run into this problem upon logging in:
No process for pid #####
I have tried reinstalling lxsession and lxsession-gtk3 to no avail.
I have been searching for days now and no resolution.
One guy in Arch said he had the problem but went away as he thought it was an upgrade to systemd that fixed.
I reinstalled systemd to no avail.
My system is otherwise completely fine and up to date; as I do a pacman -Syu more than 2 or 3 x’s a day.
Anyone have any ideas?