The commit from May 03, 2013 ("Plugin scanning: ability to use multiple threads") may have broken the postponement of previously crashed plug-ins.
In PluginDirectoryScanner::PluginDirectoryScanner, you deliberately placed recently crashed files back in the list, but in the commit above you reverse the order in which plug-ins are scanned; hence the previously crashed plug-ins are scanned first.
Also, I am making a UI for the scanning process which is a bit more elaborate. Do you think you can allow for the scanning process to optionally ignore problematic plug-ins? I want to allow the user to choose to leave them alone. A boolean in the ctor of PluginDirectoryScanner would probably be enough.