@Mayur ,
Does this issue persist only in internal users?
Sorry, we do not find a good list even if the error ID is listed, the description given doesn’t add much more information about error from it. But it seems this issue is related to conference announcement service, you can refer its code from the following link:
https://video2.skills-academy.com/en-us/openspecs/office_protocols/ms-ocer/d651f538-0e3d-44da-b457-08e3a812419a
Considering there are many factors can cause this issue, we recommend you try to follow these tips to see if can help you fix this issue:
*Update SFB Server to the latest
*Re-run certificate wizard using Skype for Business server 2015 deployment wizard as the following snapshot:
*Check if your network connection is normal in your environment.
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