Acronis True Image Backup not working

I have both a ‘full PC’ and ‘Files and Folders’ backup scheduled to run. It has been working perfectly well until today. Neither backup completes but just fails after about 30 seconds with a message that the specified file does not exist.
Checking the file structure I find that nothing is out of place and can see no reason why the procedure should fail.

This problem turned out to be router related. The ‘gateway’ icon disappeared from the ‘Network’ filer window. It was returned following a router shutdown and restart.