vCenter update manager log aquisition
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Default location of update manager
c:\program files (x86)\vmware\infrastructure\update manager

open powershell as admin
cscript vum-support.wsf /n
( /n is for only the update manager logs)

This creates a Update Manager log bundle (sometimes it goes into a .ZIP file)  :)
View the log file pertinent to the time frame of the error.