If you are getting a pop up that looks like this:
The first thing you'll want to do is quit Microsoft Lync/Skype For Business. Then go to Finder, and navigate to Go in the menu bar. When there, press the Option key (or in some cases it likes the Shift key), and that should present the Library option to appear.
Click on Library, and navigate to Library/Keychains/ and find the OC_KeyContainer_first.firstname.lastname@example.org=db file, and move it to the Trash.
Once you've moved the file to the Trash, restart your computer and relaunch Microsoft Lync/Skype For Business. The popup should no longer appear!