Arch Linux

What to you mean it can't start? Without some kind of logs, there's nothing to do here.

Same problem here. After graphical login (LXDM) I end up with openbox only.
I think it may be due to the removal of lxpolkit by lxsession 1:0.5.0-1.
Which log would you need? E.g., /var/log/lxdm.log looks fine to me.

It seems that it doesn't parse /etc/xdg/lxsession/LXDE/autostart , while it correctly parses ~/.config/lxsession/LXDE/autostart . Temporary workaround is to move every line from the former to the latter, but in case of multiple users it can be annoying.

I was facing the same issue (only openbox and my autostart programs started) and your workaround has worked flawlessly Stefano, thanks.

This is said to be the default and right behavior in their wiki (http://wiki.lxde.org/en/LXSession#autostart_configuration_file) so I don't think we will see "a fix" in the near future:

Commands globally executed are stored in the /etc/xdg/lxsession/<profile>/autostart file, and in addition, other commands can be locally specified in the ~/.config/lxsession/<profile>/autostart file. If both files are present, only the entries in the local file will be executed.

Sorry for my complaining.
I do not install any login manager, just startx to init X to Lxde. And I do not know which file contains logs.
Before the updating, I have the file ~/.config/lxsession/LXDE/autostart, and with a customise lxpanel bar. Just after the updating and startx to X, there is a black screen and can right click to open Openbox menu, that is the Lxsession is not started, but openbox works fine. After a tty2 login to check, the autostart scripts work fine too.
So I backup and delete all ~/.config/lx* setting files, the issue is the same.

less ~/.cache/lxsession/LXDE/run.log

I don't see anything strange in your log. Please contact with upstream:
https://sourceforge.net/p/lxde/bugs/

I (half) confirm this issue.
It looks like there are incompatible lxsession changes between version 0.4.x and 0.5.x.
I installed my LXDE 3 weeks ago, didn't manually edit ~/.config/lxsession/LXDE/desktop.conf (it was created automatically by lxsession after first login) and it worked. But as came lxsession 0.5, I was unable to start automatically lxsession with lxpanel and pcmanfm (as desktop manager) with their LXDE profiles.
So solution is explicitly define all important lxsession parameters in desktop.conf. Lxsession will no longer gues same previous values and will fallback to "default" profiles of lxpanel and pcmanfm instead of "LXDE" profiles which they were default for lxsession 0.4.
For explanation and working desktop.conf inspiration you can look at my imgur post: http://imgur.com/6FkxZs1
It's sad that no one catch this when lxsession 0.5 was in testing, and it looks like totally upstream issue to me too.

Thx @AloisJanicek, it works.
@City-busz, cann't you install archlinux with lxde group in virtualbox to replay this issue?

The breaking change from 0.4.x to 0.5.x appears to be as gmenguez stated: If both the global and local autostart files exist, only the local file is executed. Previously, both files were executed.

The problem of the desktop manager and panel not starting was fixed for me when I copied the 3 lines from /etc/xdg/lxsession/LXDE/autostart to ~/.config/lxsession/LXDE/autostart. So it looks like there are two solutions to the problem: AloisJanicek's solution and this one (also done by cip91).

For the original report: I can't reproduce the problem on a fresh install, so please report it to the upstream developers:
https://sourceforge.net/p/lxde/bugs/

For others: I know that LXDE does not handles updates very well, but you can ask upstream to implement something to make updates easier. Just submit an upstream feature request:
https://sourceforge.net/p/lxde/feature-requests/