If you are willing to try to repeat the problem, by all means that would be great to find another cause of it. I just looked at your error stack you posted and it did remind me of one thing that would be relevant to that. In Calibre version 0.7.49 we added code to ask the user which toolbar to add it to when installing. As a result of this, it will attempt to "load" the plugin at first install time, rather than waiting for the restart (it needed to do this so as to be able to ask the plugin what toolbars it is allowed to be installed on).
If you install an earlier version of Calibre than this, the plugin will not be properly loaded until you restart Calibre. So while you think the Install has "worked", in fact when you restart Calibre it would still have the same error while attempting to load. You just get more "early warning" on Calibre 0.7.49 onwards.
Hope that makes sense.