I suspect some MS patch caused this as no one has touched this server in months yet today, we got the following:

Server Error in ‘/Reports’ Application.

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Parser Error

Description: An error occurred during the parsing of a resource required to service this request. Please review the following specific parse error details and modify your source file appropriately.

Parser Error Message: Could not load type ‘Microsoft.ReportingServices.UI.GlobalApp’.

Source Error:

Line 1:  <%@ Application Codebehind="Global.asax.cs" Inherits="Microsoft.ReportingServices.UI.GlobalApp" %>

Source File: d:\Program Files\Microsoft SQL Server\MSSQL.2\Reporting Services\ReportManager\global.asax    Line: 1

Version Information: Microsoft .NET Framework Version:1.1.4322.2407; ASP.NET Version:1.1.4322.2407

The solution is presented in the error message. Notice the bottom line in the error message: ‘Microsoft .NET Framework Version:1.1.4322.2407; ASP.NET Version:1.1.4322.2407 ‘ – This should be running .NET 2 so we change the mapping for the applicaiton, /Reports and /ReportServer virtual folders to use V2 of .NET again (right click > properties > ASP.NET tab).