While upgrading ARA from 603 to 902, after doing the user permission synchronization, customer get following errors for 2 users:
 

25/CHRBOU7/IDM False Cannot update authorizations for 25/CHRBOU7/IDM in Automation Engine The 
Automation Engine returned an error message (4006508): Could not find the object. It may have been deleted or renamed. 
25/MORLGO3_RM/UC4 False Cannot update authorizations for 25/MORLGO3_RM/UC4 in Automation Engine 
The Automation Engine returned an error message (4006508): Could not find the object. It may have been deleted or renamed.

Those users don't exist in Automation Engine, not in the GUI and not in the OH table. They are also not displayed in the ARA interface.
The only place to find those users is in the MEMBER table of the ARA schema.

Why it happens and how to fix it?

The reason for this issue could be that those users were deleted on AE while BOND plugins was not active
(because the user sync is an one-way procedure upon actions, and it will not update periodically or base on AE user records). 

Release:
Component: ARLSMG

As long as those users are not available on Automation Engine, we can use ImportExport CLI to delete those user from ARA schema.
For detailed instruction, please find in the example below:

• First do a CLI EXPORT command to retrieve XML format output file (this step is similar to doing a select query)

ImportExportCLI.exe export -con http://localhost:80/CDA -usr 1/CDA/AUTOMIC -pwd *** -mt user -format XML -fi user_output.xml -where "system_name eq '25/MORLGO3_RM/UC4'"

• The output XML can be used to guide DELETE command, but in order to do so, you have to remove all other properties but system_name which can be used as a sole identifier.  See example:

<?xml version="1.0" encoding="utf-8"?>
<Sync xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="sync.xsd">
  <Entity mainType="User">
    <Property name="system_name" isIdentity="true">
      <Value>25/MORLGO3_RM/UC4</Value>
    </Property>
  </Entity>
</Sync>

• Then do a CLI DELETE command to perform a deletion of the selected user: e.g (simply replace "export" by "delete"):

ImportExportCLI.exe delete -con http://localhost:80/CDA -usr 1/CDA/AUTOMIC -pwd *** -mt user -format XML -fi user_output.xml -where "system_name eq '25/MORLGO3_RM/UC4'"

• Lastly please check the Member table and see if it was successfully removed.

ARA/CDA documentation: ImportExport CLI - DELETE command  (note: main type can be USER)