An error occurred during the online meeting using Lync

Dear Experts,
Our online meetings using Lync 2010 was working fine.
However, we tried having an online meeting with one of our clients, when we got this error message (see screenshot below).
Our client didn’t know how to solve this one, too.
Can someone from this forum help us?
We need to resolve this badly.
Thanks in advance.

Lync
An error occurred during the online meeting.
When contacting your support team, reference error ID 23 (source ID 242).
Troubleshooting information is available online, including best practices for using Lync.
Learn More
