Windows System Image Backup Fails to Start (error code 0x80070057)

I have a suite of Windows 7 workstations on which I now cannot do a system image backup, although I used to be able to when I first installed them. The backup target is the D: drive of one of a pair of 2008 R2 Foundation servers (either one). Once I've selected the target server and drive from the "Select a Network Location" dialogue, and entered appropriate credentials there, back on the "Where do you want to save the backup?" dialogue, when I click the "Next" button, I just get a message box "An error has occurred: The parameter is incorrect. (0x80070057). Close Windows Backup and try again". Needless to say, that doesn't help.

I've come across many threads on this and other forums with similar problems, but none of the suggestions I found (like setting the locale to use '.' as the decimal place, increasing the virtual memory to a preposterous size, or running CHKDSK) have helped.

I know malware is not the problem since these workstations and the servers are on a closed network with no external connections at all, and all removable media are disabled.

Configuration:
Workstations:
HW: Dell Precision T3500
HDD: RAID 1 mirrored array of Seagate Barracuda 500GB SATA II
NWIF: Intel PRO/1000 PT link aggregated dual 1 Gbaud NIC
OS: Win 7 64 bit Pro SP1

Servers:
HW: Dell Poweredge R210
HDD: RAID 1 mirrored array of Seagate Barracuda 250GB SATA II
NWIF: Broadcom dual 1 Gbaud on MBrd
OS: Server 2008 R2 Foundation

OK, folks, riddle me this if you can:

I fixed this problem as a by-product of fixing another one, apparently unrelated, which was that I couldn't access the workstation's DVD drive - I just kept getting the dreaded "Access is denied" mantra. In my searching, I came across a suggestion to uninstall the drive and reinstall it. Seems kinda extreme, but it would certainly initialise its security settings.

Well I did it, and sure enough I now have full access to the drive, but then when I opened the Backup toolbox to create a new system repair disk, I found the backup to my system servers is now working, along with correctly identifying the DVD drive and an external USB HDD I have connected as usable backup devices (which it didn't before).

It makes about as much sense to me as getting your TV to work by turning the kitchen tap on, but there you go - that's Windows for you.