Just wanted to post an update. This problem occurred again. It's been 10 days since it last happened. Last night everything seemed fine. This morning I start Revit and open the file I was working on last night, and I get the "improper argument" error. Revit won't open any files.
I'm going to try the repair through control panel again. One problem with this fix is it's basically like a new installation, and all customizations are lost. And I don't know yet how/where the Revit customization file is or how to save/replace it.
I have not installed 2014 yet, this is on 2013.
So infuriating as I have a submission deadline in 3.5 hrs.