Ok -- I've been chasing this one down and now I found the common link.   When I have a dash (_) in a file name I get this message:

CGI Error
The specified CGI application misbehaved by not returning a complete set of HTTP headers. The headers it did return are:

If I try a page that doesn't have the dash, it works perfectly.   I've tried this and modified my page names, and that solves the problem.