Symptoms
Provisioning of new Office 365 CSP Tenant or any other Office 365-related operation (attempt to sync, check for domains availability) fails with the error:
2016-07-08 10:21:27,664 [ 8] ERROR aps_endpoint: TenantsController.Post finished by exception Microsoft.IdentityModel.Clients.ActiveDirectory.AdalServiceException: AADSTS70002: Error validating credentials. AADSTS50012: Invalid client secret is provided.
...
2016-07-08 10:21:27,679 [ 8] ERROR aps_endpoint: System.Net.WebException: The remote server returned an error: (401) Unauthorized.
...
500 (Internal Server Error)
Cause
Incorrect reseller credentials in Office 365 application instance settings.
Resolution
Check with Microsoft that all the CSP credentials are valid and not expired.
Go to Microsoft portal https://partnercenter.microsoft.com/en-us/pcv/apiintegration/appmanagement and check if any of application secret keys have an expiration period close to today. Generate a new Key if so, update expired one with this key in O365 application instance settings.