Hi Ronald v Ackooij,
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:
https://support.microsoft.com/en-us/kb/929135
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.
Regards,
Alice Wang
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