Acronis True Image: Entire PC backup fails with "Unknown status" if Windows Sandbox is enabled

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.