Experienced a similar issue on a T620 today.
If you try to upgrade the agent via HPDM, the task will show successful, but agent will go offline and if you check on the device itself, it will still have the older version.
Only workaround I have at the moment, is to copy the 2 update files and execute the agent upgrade manually.
EDIT: the same error message is shown during the upgrade, but the upgrade will still be OK after you close the error message.