If you get errors about VSS failing to create a snapshot, and you have not changed anything on your server recently, it's a good idea to check the presence of a lot of old shadow copies, taking up space. Although old copies will be purged automatically, the VSS Writer doesn't wait around for it, and fails the backup. So try deleting shadow copies or increase the shadow copy storage limit.
A backup may also fail because of a full external HD, or when the HD contains incomplete or corrupt backups. Try using another external HD. If it works, the problem is with your external media, and not with your server or with WSB.