Microsoft KB article below involving Security Patch: MS15-027.
Try to map into the Storage Sync share to verify if you're affected by this known Microsoft patch issue. If you receive a NT_STATUS_LOGON_FAILURE, the recommended workaround would be to either uninstall the security patch or perform the following changes via Group Policy (if applicable):
Configure [LAN Manager Authentication Level] to accept [Send LM & NTLM responses] by setting the following group policy:
Computer Configuration >> Windows Settings >> Local Polices >> Security Options >>Network Security: LAN Manager authentication level >> Send LM & NTLM responses