External user cannot sharing application or whiteboard

Dear all,

I've encounter problem in setting the Desktop/Application and whiteboard sharing for external users. Internal users work like a charm.

I've a Lync server and FE server all behind firewall and FE is NATed.

Error message is as below:

An error occurred during the online meeting.

When contacting your support team, reference error ID 530 (source ID 241).

Youhelp would greatly be appreciated. Thank you.

Best Regards,

Lewis