Windows 8 64 bit DAta sync agente ver. 4.0.89.0
Error message , I tried reinstall 3 times data sync and 4 tiems new agent and new key also.
No prosy or internet fire wall one no other fire wall software also.
Please enter a valid agent key to proceed. The key must be generated from SQL DAta Sync web portal
agent key 16f25226-2a20-49c3-acfc-062057c42b41:gWpFO0NHm4Xq9bw7fLzo8YS49iSowEDQToYpbMCKqFk=
id:LocalAgentUX_Info, rId:, sId:00000000-0000-0000-0000-000000000000, traceLevel:Warning, version:4.0.0.0, methodName:AgentModel_AgentStatusChanged, message:Agent UI stopped polling for the latest status from Microsoft SQL Data Sync Windows service due to errors. Please address any errors displayed in the UI and then restart Microsoft SQL Data Sync Windows service on your machine and Agent UI to resume polling.
This is a common error. You have to change your system's date to match azure server's time.
Go to the Azure Website and check the location of the server, than set your system's date and time to the same location's time zone.
This worked fine for me,
来源：https://stackoverflow.com/questions/16593671/sql-azure-data-sync-agent-invalid-key-error-mesage