Hi charlesallen-1636,
What do you do before this issue happened?
Do all the users have the same issue?
What is the client version?
It recommends affected users update the Office app by open any Office app and choose File > Office account > Update Options > Update now.
Also, you can try to repair Office by this link:
https://support.microsoft.com/en-us/office/repair-an-office-application-7821d4b6-7c1d-4205-aa0e-a6b40c5bb88b.
Then, perform a clean boot on the machine as this article:
https://support.microsoft.com/en-us/help/929135/how-to-perform-a-clean-boot-in-windows.
Moreover, you can check if there is any related error message in Skype for Business front-end server event viewer.
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