Maintenance Plan Backups failing since KB4013106 installed
Edit the maintenance plan and change the credentials used for Local server connection to Use Windows NT Integrated Security.


Open the Maintenance Plan in Design view.
Click Manage Connections
Select Local server connection and click Edit
Under Enter information to log on to the server, choose Use Windows NT Integrated security
Save your changes
Execute the plan/schedule to verify that this has cured the problem


No idea what causes this issue, but for us, it seems to have been directly after Hotfix 4422 for SQL Server 2016 (KB4013106) was installed automatically by Windows Update.