SSRS Report error – Assemble expression_host

When trying to run an SSRS report the following message was returned:

Failed to load expression host assembly. Details: Could not load file or assembly ‘expression_host_1c6a722005524f70be7333e2c928f09a, Version=12.2.3000.0, Culture=neutral, PublicKeyToken=null’ or one of its dependencies. Failed to grant permission to execute. (Exception from HRESULT: 0x80131418) (rsErrorLoadingExprHostAssembly)

The problem is an execute permission, but not one that relates to user permissions. When setting up the SSRS server there are a number of config files that are often changed, depending on your system configuration. One of these changes is to change the permission set name from execute to fulltrust ) PermissionSetName=”Execution” to PermissionSetName=”FullTrust”). However this setting is case sensitive so if you enter anything other than “FullTrust” you will get the above error.