?
Based on the error message and symptoms (it was working fine, until it wasn't), it would appear that the indexing error is from a poison package.
We made the mistake of using Microsoft’s Packaging libraries (System.IO.Packaging) to open this package, and among many other bugs, it will produce an un-catchable exception (fun fact: if you happened to be on a non-english Locale, it will terminate the whole process with a stack overflow exception).
We cannot work around this problem, but the bug appears to be fixed in .NET 4.5.1. If you upgrade to that, the error should go away.