Comment 2 for bug 1233330

That's a strange error. It will likely be realted to something in your
calibre setup. First step is to disable any third party plugins you
might have in Preferences->Plugins and then see if the error goes away.

If you can think of anything else in your setup that might be
unusual/different from the default, it would help.

ANother thing that would help is if you could identify an exact set of
steps that causes the problem with high probability.

 status incomplete