I am using the trial version of the paid Macrium Reflect version 7

I get the following message when attempting to create a differential backup:
Completing the backup (BackupComplete) ...
(Waiting for the asynchronous operation to finish...)
(Waiting for the asynchronous operation to finish...)

I what I assume is the VSS log file there is this message:
Date     8/3/2018 Friday 12:19:25 PM
Type     Warning
Event     6016
Source    VSS

Express Writer error: failed while adding express writers from directory System.

Operation:
 Initializing Writer
 Gathering Writer Data
 Executing Asynchronous Operation

Context:
 File Path: C:\WINDOWS\Vss\Writers\System\
 Execution Context: Requestor
 Current State: GatherWriterMetadata

Error-specific details:
 Error: CreateFile("75DFB225-E2E4-4d39-9AC9-FFAFF65DDF06.xml"), 0x80070002, The system cannot find the file specified.

I can make full backups.