If you have ever encountered this error, you may find this post helpful. It frequently occurs when one tries to join an external meeting, and one's Lync (server) does not permit access by the external party who sent you the meeting request, or vice-versa.
By default, if you have Lync installed locally, clicking on the "Join Online Meeting" hyperlink will try to initiate the meeting using your Lync client. If your Lync administrator has enabled external access via federation or other means, this error will occur.
Fortunately, a simple workaround exists:
1. Right-click the link in the original invitation, and "Copy Hyperlink"
2. Paste the URL into a new browser tab
3. Append the suffix "?SL=1" to the URL
The "?SL=1" switch in the URL forces the meeting/session to use the web client, rather than the local client.
There are, or course, some limitations to using the Web Client vs the local client, but if your local client cannot access the meeting, this is a simple, quick workaround you can try so that you can participate in Lync meetings with external parties. You can also use this tip to help external parties to connect to Lync meetings that you set up, and it's remarkable how much more productive we can be when the first ten minutes of a meeting aren't occupied by "Can you see my screen? etc.)