There are a few different possibilities as to why you are getting this error message.

a) your disk is almost full and there isn't enough space to write the backup file in the selected destination location.

b) The program Microsoft Application Compatibility Toolkit 5.6 is installed on the workstation.

c) Backup file is being saved to a folder using a mapped drive.  (try selecting the UNC path instead)

d) The network share location where you are trying to save the archive folder does not have full network permissions.

This, or a similar error can be returned when one or more of the files included in the backup are greater than 2GB in size.

If a file in the backup is greater than 2GB in size, is there anyway it will allow the backup?  The odd thing is, they have done this backup every week for years without incident, and all of a sudden it's starting to give them this error.  The hard disk space is fine, it's only about half full.  They have full admin rights to save to the folder, etc.  It is using a mapped drive, so I am testing the UNC path instead.  Will notify if that works.