Sys.WebForms.PageRequestManagerServerErrorException: 500

Sys.WebForms.PageRequestManagerServerErrorException:An unknown error occured while processing the request on the server.The status code returned from the server was:500

The real error was:
Invalid postback or callback argument. Event validation is enabled using;<pages enableeventvalidation=”true”/>; in configuration or ; in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.

The fix for me:
EnableEventValidation=”false”