Case Resolution

Hi ,

 We are using  CRM 2015 Online. When a user tries to resolve a case using the external web address for crm online (xycrm.mycompany.com) they can resolve a case successfully. But when resolving a case  using the internal web address (crm.mycompany.com) they cannot resolve a case , due to the Case resolution page not appearing.

Why could be the cause of this?

Thanks