Mists of Pandaria Beta - Build 15650

A new update came out this morning, WoW: MoP Build 5.0.1 15650 (BETA). Note that this patch is not yet pushed to the launcher, so you can't *normally* download it. Bluepost about that - here.

In general this *should* fix now the nasty stability issue that we experienced with the previous beta patch. Also it includes a lot of new PvP stuff - PvP trinkets, PvP Sets (rogue&shaman), PvP weapons, as well as several items  and achievements. As usual almost all classes have some modifications regarding their spells.

As usual you can read all about the new changes at MMO-Champion. (seems like they are the only ones to have information about it until it's officially out on the PTR)
As soon as I can, I'll post links to WoWHeadNews an several other blogs.

How to fix WoW Launcher: "Error: The file Localization.xml" could not be loaded."

Right click on the Launcher.exe or the shortcut and select Run as Administrator.
Done! That worked for me.
If it doesn't work for you, you can delete the Cache folder (World of Warcraft\Cache) and try again.
Another way would be to close the Agent.exe process.