After offline investigation there seemed to be a mistake in the DMS.xml. The failover agent was present in the DMS.xml as failover agent and as a clustered agent at the same time which is incorrect. Updating the DMS.xml on both main and failover agent and performing a DMA restart solved the issue.

The cause of the issue was because that agent was first added into the DMS/cluster, removed again and added as failover afterwards while the DMS.xml was not cleaned up yet.