There is an undocumented assumption baked into SharePoint Search that the Default Public URL of a Web Application will be crawled. If you want everything to work auto-magically, crawl the Web Application's Public URL for the Default zone. Otherwise, when crawling a non-Default zone, expect things to break. Read here to understand why.
As I began in part 1, seemingly “sporadic” query problems are often just straightforward failures being masked by the three levels of load balancing involved with a SharePoint 2010 Search Query and my goal here has been to help unravel all the moving pieces. Here, I described the load balancing that occurs when there are multiple instances of the SQ&SS (e.g. Farm level load balancing) and from this, begin to explain why the "Internal Server Error Exception Occurred" failure is completely ambiguous.