Acronis True Image 2018: backup cannot be started, pauses, or fails with the "Ensure that folder is accessible." error

Symptoms

In Acronis True Image, you have a backup task of your computer of some files. The backup should be saved on a NAS device or in a shared folder on another computer.

You try to start the backup task, but it does not start. Or the backup task pauses or fails. You see this error message:

Ensure that folder <folder_path> is accessible.