Can you have 1.1,2.0,4.0 on the same server ?
I have an app written in 2.0 that works fine, if i even SIMPLY install 4.0 and you try to start the site the app pool dies. I installed 4.0 with the /noaspupgrade option, verified that in the mappings that aspx files are mapped to 2.0, no 4.0 are shown there. Once install of 4.0 it fails with following eventlog info;
The HTTP Filter DLL c:\windows\microsoft.net\framework\v4.0.30319\aspnet_filter.dll failed to load. ...
eventid 2214
source w3svc-wp