Did this issue only appear on the specific SFB client?
For this issue, please try the following steps:
1. Delete SFB cache files and test again. (%userprofile%\AppData\Local\Microsoft\Office\15.0\Lync\sip_UserName@Domain.com)
2. Try to perform a clean boot on your computer:
3. If the issue persists, please try to repair office OR reinstall SFB client, then test again.
For SFB server side, please try to install the latest update for it.
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