If that error message is from the job backup summary, the detail that’s needed is missing. If it’s from reporting options such as email, see if any other log details are at the bottom. If not, you could try watching it live using About → Show log → Live, or use the job’s Commandline option, or set job –log-file. Example of more detail:
Backups suddenly failing - 4 machines now
System.Exception: The file duplicati-20181122T130004Z.dlist.zip.aes was downloaded and had size 2097152 but the size was expected to be 2124781 at
and then more, but this is one of those errors (and there are others) where one more line would really help…
Verifying backend files is the step after the backup that this is from. The name changes because it’s random.
The TEST command is the same operation if it ever becomes necessary to survey your files heavily by hand. You could convert the default GUI Commandline backup format to run this command, if you want a direct test.