Hello Lars,
Please, check the web.config if you have this handler in /configuration/system.webServer/handlers:
<add name="ScriptResource" verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" preCondition="integratedMode" />
and this module in /configuration/system.webServer/modules:
<add name="ScriptModule" preCondition="managedHandler"
type="System.Web.Handlers.ScriptModule, System.Web.Extensions, Version=3.5.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />