Hi Martin,

In order to prevent backup from failing, you can use one of the following methods to eliminate this error message during future backup operations:

Method 1:Create the missing folder "C:ProgramDataBlueStacksUserDataLibraryMy Apps", as listed in the error message.

Method 2:
Exclude the folders from the files to back up in the Backup settings, and then include those folders in their original location. To exclude the folders in the Additional Locations and to include those folders in the original location, follow these steps:

1.In Control Panel, open Backup and Restore.
2.In the Backup section, click Change settings.
3.In the Set up backup dialog box, click Next.
4.In the What do you want to back up dialog box, click Let me choose, and then click Next.
5.Expand Data Files, expand <var><your user name></var> Libraries, expand Additional Locations, and then click to clear the AppData folder, Contacts, and Searches check boxes.
6.Expand Computer, expand the item for your system drive (for example, expand Local Disk (C:)), expand Users, expand <var><your user name></var>, and then click to select the AppData,Contacts, and Searches check boxes if they exist.
7.Click Next, and then click Save settings and exit.
8.In Backup and Restore, click Back up now to perform a new backup. The backup operation should complete without errors.

I hope this helps.
In case still you don't get the backup completed, revert back to me and I will provide another working solution at the earliest.

Regards,
Lokesh