Please tell us the time set for your synchronized backup (your original backup, not the tests). You said that for the second test of your Documents folder, the synchronized backup failed, although for the first test it did the the job. Have you restarted your computer after you created the second test synchronized backup?
The following was some days ago so I'm working from memory, and I've done many tests by now:
In the first synchronized backup of the Documents folder the time interval for the sync run was 5 minutes and once I saw that the synchronized copy worked, I deleted it (editing the synchronization job is not possible) and created a new synchronized backup of the Documents folder with the time set being 1440 minutes (i.e. once every 24 hours). The computer would be shut down and turned off at night. This Synchronized Backup at intervals of 1440 minutes did not work the next day (i.e. after 24 hours it was created).
In another test the synchronization backup failed but I'm sorry I can't remember now how many hours after the previous success it stopped doing the sync run.
I uninstalled Comodo Backup, restarted my computer, and re-installed Comodo Backup and did a test of the synchronized backup of the Documents folder with the time interval being 5 minutes and the sync run would work, but it would stop doing the sync run after about 8 hours, and the computer was not shut down (turned off) during this test.
Instead of uninstalling Comodo Backup and re-installing it, I now only need to delete the Backup folder and also delete the synchronization job in "Schedule" tab and then create a new Synchronized Copy backup of the Documents folder and the sync run begins to work again, but then stops working after about 8 hours.
I've now started a new test of a synchronized copy of the Documents folder at 5 minute intervals and it is working by successfully doing the sync run every 5 minutes. I'll let you know how this latest test develops.
Thank you for your help.
EDIT: Although the synchronization copy is working for now in the current test, I'll just add that in the "Schedule" tab the "Last Run" always shows "Not available", but "Last Run Status" shows "Success".