Ok, it's actually simple one :) And it's not a bug.
@fdev31, you could come into IRC #blenderwiki, and we can try resolve your commit access issues.
No problem, thanks for quick headup :)
You do realize that sensationalist statements like that or comparisons to Houdini are not going to make this happen faster if the algorithm we use can't support this with an easy patch, right?
Well, I'm sorry for waisting your time, but after checking again it looks like the crash has been caused by having mira_tools enabled without copying the folder to the addons_contrib folder.
Fix bad import in del context, and fix/enhance ui_previews template to only rescan when dir changes.