The error you're encountering in System Center Operations Manager (SCOM) with Event ID 31555 typically indicates an issue related to data synchronization between the Management Server and the Data Warehouse. The error message suggests that there's a problem with the data being forwarded to the Data Warehouse, particularly related to Maintenance Mode workflows.
Your investigation into the Maintenance Mode schedules and removing any null objects is a step in the right direction. However, it seems there might still be some underlying issue causing this error to persist.
Here are a few additional steps you can consider to troubleshoot this problem further:
Review Workflow and Configuration: Check the configuration of the "Microsoft.SystemCenter.DataWarehouse.Synchronization.MaintenanceMode" workflow. Ensure that there are no misconfigurations or inconsistencies in the way it processes Maintenance Mode data.
Check Data Consistency: Validate the consistency of the data being sent to the Data Warehouse. Ensure that the data being generated is in the expected format and within the range of valid values. Data inconsistency might trigger the ArgumentOutOfRangeException you're encountering.
SCOM Database Health: Check the health of the SCOM database. Any database corruption or inconsistency can lead to such issues. Running database maintenance or integrity checks might be helpful.