Symptom
Entire PC backups in Acronis True Image started to fail with error "Unknown status" after Windows Sandbox feature of Windows 10 was enabled.
Windows Sandbox is an optional feature in Windows 10 that appeared in May 2019 Update (Version 1903). It is not enabled automatically. One has to mark the corresponding checkbox in Windows Features list.
Example 1:
Example 2: 
Example 3: 
Cause
Known incompatibility issue with the virtual disk that Windows Sandbox uses for its operations.
Entire PC backup automatically includes all internal disks into the backup source. Backup fails, when the program tries to access the virtual disk of Windows Sandbox.
Solution
The issue is expected to be fixed in one of the next versions of Acronis True Image.
In the meanwhile, there are workarounds:
Method 1. Turn off Windows Sandbox
If you turned on Windows Sandbox just to check out the new feature of Windows 10 and then decided that you do not need it, disabling Windows Sandbox would fix the backup.
- Open Windows Start menu
- Type optionalfeatures together, without a space between the words optional and features, and press Enter on keyboard.
- Scroll down to "Windows Sandbox" and uncheck that option:
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- Click OK.
- Click Restart now to reboot the computer and complete turning off Windows Sandbox.
Method 2. Exclude the virtual disk of Windows Sandbox from the backup
Because 1) exclusions setting does not accept disks as exclude rule, and 2) changing backup source of Entire PC is not supported, this method consists in recreating backup plan from scratch. A new full backup would be created as a consequence.
- Delete existing backup task: open the backup menu and select Delete.
- Create a new backup task, selecting "Disk and partitions" backup type.
- Mark all the disks, except for the disk "MSFT VIRTUAL DISK 1.0" and disk volume "PortableBaseLayer" as displayed on the picture below:
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- Click OK.
- Set other backup options and finish recreating the backup plan.
