MSCRM 2013 Migration: Custom entity form issues

We have migrated to CRM 2013 from CRM 2011. we are facing the following issues post Migration.

Onload of one of the custom entity form, it was giving error in the internal javascript(JsProvider.ashx....).

Error-Line: 2 Error: Object doesn't support property or method 'get_disabled'

And also noticed that While saving the form customization of the same entity getting error permission denied. The error was on the linewindow.top.opener.auto(Mscrm.EntityTypeCode.SystemForm).

Couldn't understand why this is happening? The user has Administrative privileges. Have anyone faced this kind of issue?

Note- THis entity has multiple forms and this issues are happening only with two forms.

Looking for your support.

Thanks.

Nithya