I reported the same error in another thread (https://forum.emclient.com/t/saving-and-restoring-favorites-and-delegated-accounts/100939/7).
Here is a lightly edited version of the post:
I think I have found the issue, and it points to a bug.
(I had previously deleted the backup task in Windows Task Scheduler) When I tried to re-create the scheduled backup within eM Client, it created the task in the scheduler. However, what it created in Task Scheduler was, in Properties → Actions → Edit:
Program/Script: C:\Program Files
Add arguments (Optional): (x86)\eM Client\MailClient.exe/dbbackup -backup -silence
In other words, it had wrongly separated the information. Not surprisingly, when I tried to run this, it failed. So I changed it to:
Program/Script: “C:\Program Files (x86)\eM Client\MailClient.exe”
Add arguments (Optional):/dbbackup -backup -silence
and that runs on demand. This task has now run successfully and created a backup.
The bug is in eM Client setting the wrong values in the task - it looks like it may not be using quote marks when populating the Program/Script field.