zip is corrupted on Windows Server 2012 and IIS 8.5

I have asp.net code using SharpZipLib version 0.86.0 that creates a zip file and returns it to a client.  The code has worked successfully for years.  We recently upgraded servers to windows 2012 and IIS 8.5 and now the code produces a zip file that cannot be openned.  The error we get is "The compressed (zipped) folder is invalid".  I have tested it extensively - a console app that doesn't go near IIS works just fine.

Any ideas what might be happening?