We moved from a windows 2k3 server to a 2k8 server and are having problems.  The install guide taks about changing the app pool to classic .net.  The install creates the iMIS15 and public apps under the default web site in IIS.  We changed the app pool there and all works well.  We then created an appliation called iMIS15 under our web (imis.association.org/imis15).  on the same server.  When we set the app pool on that application to classic .net, we get a version miss match error when trying to open the desktop and errors in the web browser we get a 500.21 error:

HTTP Error 500.21 - Internal Server Error

Handler "Plesk_Handler_09196967" has a bad module "ManagedPipelineHandler" in its module list

this is driving me mad!  Any ideas?