MSI 1603 error code indicates that you must identify what installation task gave a return code of "03"
DLP upgrades suppress the MSI reboot by incrementing and decrementing the RESTARTMANAGER counter.
RESTARMANAGER calculates the initial value of this counter when MSIEXEC launches based on several factors, it's initial value is not determined by the DLP installation or upgrade process.
RESTART MANAGER: Will attempt to shut down and restart applications in no UI modes.
InstallValidate. Return value 3.
INSTALL. Return value 3.
This indicates a reboot was already pending.
This indicates a pending reboot caused the agent upgrade failure. If a reboot was already pending prior to the upgrade starting this can prevent certain file activities and cause the upgrade to fail.
Here are some common registry enteries that may suggest a reboot is already pending