Do you know on which specific tabcmd that you get the 406? There could be multiple reasons such as:
1. Insufficient permissions to the workbook that the userid is trying to download
2. I vaguely recall getting this error a while back if the workbook to be downloaded had no data. I'm not sure if this is still true or not
Thank you Jeff for looking into the issue , sorry for the late response , i was on vacation last week . It has been resolved now and the issue is one of the workbook's data source was published with "Prompt User for Password" which has been changed to "Embedded Password" .