My website was in initially at target framework 3.5, and i have changed it to 4.5, everything works fine , when i click button on a page where script manager is added it shows following exception:
Sys.WebForms.PageRequestManagerServerErrorException: Sys.WebForms.PageRequestManagerServerErrorException: An unknown error occurred while processing the request on the server. The status code returned from the server was: 500
whenever i click this compiler button in my page it shows the above pasted(screenshot) error, this error comes in ScriptResource.axd
try adding below line in your httphandlers and handlers section in web.config file.
Please comment here and let others know if and when issue is resolved.