Symptoms

Entire Mac backup or Disks backup fails with the following error in log:

 error 0xfff1: Resource deadlock avoided

Path mentioned in error log is similar to:

path: /System/Volumes/Data/Users/<username>/Library/CloudStorage/...

Cause

The folder /Users/[username]/Library/CloudStorage/<XXXXXX> is the folder for iCloud files on Mac. These files are stored on the cloud rather than on the local machine.

Solution

Add CloudStorage folder to exclusions: 

1. Select your backup task in the list and click Settings

   

2. In settings window, go to Exclusions

   

3. Click on "+" button, then select Exclude specific file or folder. Browse to the folder /Users/<username>/Library/CloudStorage/<....> that is mentioned in the error log, select it and click Exclude to add this folder to the list of exclusions.
4. Click OK to save changes.