During Exchange 2010 application aware backup with “Copy only” option enabled you notice following events in Application event log on Exchange server:
Log Name: Application
Date: 5/10/2018 5:38:39 PM
Event ID: 9782
Task Category: Exchange VSS Writer
Exchange VSS Writer (instance 7333fe6c-94b5-4d0f-b62f-517eec18e04d:2) has completed the backup of database '<Database name>' with errors. The backup did not complete successfully, and no log files were truncated for this database.
<Provider Name="MSExchangeIS" />
<TimeCreated SystemTime="2018-05-10T14:38:39.000000000Z" />
Copy only backup does not perform transaction logs truncation which may result in the mentioned event being generated.
Event can be safely ignored.