Access is denied at System.Diagnostics.Process.StartWithCreateProcess(ProcessStartInfo startInfo)

I am trying to upload files and convert them to another format before saving them on my web server, but I get the following error:
System.ComponentModel.Win32Exception (0x80004005): Access is denied at System.Diagnostics.Process.StartWithCreateProcess(ProcessStartInfo startInfo) at System.Diagnostics.Process.Start()

Patterns / design suggestions for permission handling

We have a rather complicated system of permission handling in our (ASP.NET web) application. Users can have specific permissions on different kinds of objects, some permissions are even packed into groups / roles that are assigned to users. All in all this ends up in a pretty complicated mess where for determining whether a user can do / see something you have to evaluate many different sources of permissions and this is done somehow on-demand and based on specific situations.