Back in October we upgraded one of our sites (on IIS 6) from .net 3.5 to .net 4.0. Since then we have been getting the following warning and error pretty much everyday: Warning: ISAPI 'c:\windows\microsoft.net\framework\v4.0.30319\aspnet_isapi.dll' reported itself as unhealthy for the following reason: 'Deadlock detected'. Error: It is not possible to run two different versions of ASP.NET in the same IIS process. Please use the IIS Administration Tool to reconfigure your server to run the application in a separate process. We have checked all applications and ensured that they are running on the correct versions of ASP.NET and also in their own separate app pools. We have looked at the IIS logs at the time the errors are recorded but this doesn't give any clues as to the issue. Is there a tool available that could give us more information as as to which application is causing the errors and what was happening at the time?