I'm using the most recent version of Macrium Reflect Free (because I don't need the additional capabilities of the paid versions) in my Windows 10 machine.  Since the Creator's Update to W10 was installed, I cannot create an image of my hard disk (which I create on an external hard drive).  A minute or so after the backup copy begins, I see a failure message "... cannot find the file specified", referring me to the log for additional detail.  However, the log does not say what file had not been found, or even seem to indicate the failure.

Is this only when scheduled backups run, or do you also see this error when you right-click your definition file and choose Run Now?  Have you tried creating a new definition file from scratch?