Could not find the following files: [BLANK]

[coverton]

Platform / OS: Windows 7
KeyShot version: 6.3 Pro Floating License 64 bit
Steps to take to reproduce the problem: Render > Queue > Add Scene Sets > Process Queue

When I first tried to render a batch I received "Could not find the following files: [two missing materials]".  I tracked them down, loaded them, restarted KS and tried to start another batch.  Now I'm receiving the same error but without any files listed (screenshot attached).  What can I do to fix this?  I realize I can just click 'ignore' for the time being but I have hundreds of renders to create and I simply can't babysit the queueing process to dismiss a dialogue every 5 minutes or so.

Thanks in advance for any help!

EDIT:  I forgot to grab a screenshot but I got similar error when trying to save as a new package.   Said it couldn't find a file but the dialog was empty otherwise.