How to investigate the root cause of a Failing status on the Failover database status.
The reason shown is: object reference not set to an instance of an object.
Is there a log file with more info on what caused the error?
I would start the investigation by looking at the following two logfiles on your DataMiner server: SLDBConnection and SLDataGateway.
I believe you will find your first hints on what's going wrong in here.