TaskNotFound for new assignments

I'm trying to submit new assignments programmatically
('QueueAddToProject') to Microsoft Project Server 2007. However I get
a queue error ('GeneralQueueJobFailed') with error description
'TaskNotFound'. In the error details the UIDs for the tasks not found:
a) are not submitted by our code to Project Server
b) do not exist at the Project Server (I checked the MSP_TASKS tables
on Project Server databases).
Has anyone faced a similar problem before?