Try this, from WebForms UnobtrusiveValidationMode requires a ScriptResourceMapping for jQuery:
Remove below in web.config if you have
<add key="ValidationSettings:UnobtrusiveValidationMode" value="WebForms" />
or
registering jQuery in Global.asax in the Application_Start event like this:
ScriptManager.ScriptResourceMapping.AddDefinition("jquery", new ScriptResourceDefinition { Path = "~/scripts/jquery-1.7.2.min.js", DebugPath = "~/scripts/jquery-1.7.2.min.js", CdnPath = "http://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.4.1.min.js", CdnDebugPath = "http://ajax.aspnetcdn.com/ajax/jQuery/jquery-1.4.1.js" });