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Hello Christian
Thank you for your help, the problem is now fixed. I did the following:
Stopped the Message Router, Agent, Update Manager and Management Service services in that order.
Emptied the Warehouse folder located at C:\ProgramData\Sophos\Update Manager\Update Manager\Warehouse.
Started the same services in the same order.
Forced SUM to 'Update now'.
After a short time the error message disappeared from the console and the Warehouse folder was populated with files bearing today's date.
Thanks again!