Dedupe engine offline

I did find a thread talking about the "Dedupe engine offline" error, but still could not get the problem resolved, so started a new discuss here.

In DLO Agent, when it was trying to backup, got error "Dedupe engine offline", then could not work at all. I searched found a discussion thread says the reason could be:

- DLO client machine is not connected to network
- DLO clientNot able to access NUDF/DSL
- DLO client Not able to connect to dedupe server.

But I double checked, the DLO client machine is connected to network, and have can access the NUDF and DSL, and https://DLO server:8443 got "StoreSmart Dedupe Server Status: (20159) Active. So the DLO client can connect to the dedupe server.

How can I get rid of the  "Dedupe engine offline" error?