3620825 - IPS job failing with the error "backendKeys" is null

The IPS job fails during execution, and the job logs contain the message: 

Cannot execute provisioning job in tenant context: [tenant-id]
Caused by: Error during execution on behalf of tenant with ID: [tenant-id]
Caused by: Can not read entities from source system: '[system-id]: XXXXXXXXXXX' 
Caused by: Connector cannot process more entities due to irreparable error 
Caused by: All target systems are marked as invalid due to the following reasons: XXXXXXXXXXXX - Cannot invoke "java.util.Set.iterator()" because "backendKeys" is null]

error=Unable to write entity to target system with name XXX Caused by: Cannot invoke "java.util.Set.iterator()" because "backendKeys" is null,

Identity Provisioning System

"backendKeys" is null, Cannot invoke "java.util.Set.iterator()", IPS, All target systems are marked as invalid , KBA , BC-IAM-IPS , Identity Provisioning Service (IPS) , Problem