Unable to Add/Remove/Modify contacts in Skype for Business

Prerequisites

Ensure that Skype for Business license is turned off for affected users in Office 365. See Add/Remove Office 365 Licenses for assistance. 

Process

1. Close Outlook
2. Close Skype for Business
3. Open Task Manager
4. Verify all instance of Skype for Business, Lync, Microsoft Skype for Business, Microsoft Lync, Outlook and Microsoft Outlook are closed. 
5. Open the File Explorer
6. Navigate to %userprofile%/appdata/roaming/microsoft/Office
7. Select the corresponding version of Office
8. Select Lync
9. Delete the AccountProfiles.dat folder

Clear Skye for Business cache

To further improve performance, the Skype for Business cache should be cleared. See How To Clear File Caches

Open Skype for Business and test.