Hi D_Goossens,
Agree with KHart85.
Did this issue only appear on the specific user? If the issue only appeared on the specific user, please try to compare user attributes between affected user and other normal users.
If all users had this issue, may be it is caused by the integration between the SFB online and exchange server, please refer to the following document to check the integration
https://support.microsoft.com/en-us/help/2614242/how-to-integrate-exchange-server-2013-with-lync-server-2013--skype-for
Moreover, our forum is focus on SFB on premise, because you use SFB online, so we suggest you post in SFB online forum, there are more experts will help you with this issue, thanks for your understanding and support.
https://answers.microsoft.com/en-us/msoffice
Regards,
Alice Wang
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