Hi Mick,
Welcome to post in our forum.
Did the issue only appear on the specific SFB client?
If the issue appeared on the specific client, please try to delete SFB cache files and make sure SFB client with the latest update, then test again.
If all users had this issue, please check the steps for configuring EWS for SFB, the following document is for your reference
http://communicationsknowledge.blogspot.sg/2013/07/exchange-web-services-was-not-showing.html
https://technet.microsoft.com/en-us/library/jj688098.aspx
Note: Microsoft is providing this information as a convenience to you. The sites are not controlled by Microsoft. Microsoft cannot make any representations regarding the quality, safety, or suitability of any software or information found there. Please make
sure that you completely understand the risk before retrieving any suggestions from the above link.
Regards,
Alice Wang
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