Failed to open a session for the virtual machine

[mykol_j]

Over past year, I'll be working along fine, periodically creating snapshots, cleaning up old, nothing too cosmic, and wham! Will get "Failed to open a session for the virtual machine...{then the name}". It claims it could not open the medium C:\Users\{path to the .vdi}. Doesn't matter which snapshot I select. This has happened at least a half-dozen times.

Every time, the path is fine, nothing's changed in the config of VirtualBox, it doesn't seem to coincide with any activity such as deleting a snapshot, or creating a new one. The .vdi it's pointing to can't be found in any of the snapshots or anywhere in the config file (*.vbox). I've read some of the posts here that allude to somethings like this, and I tweek the .vbox file and monkey with the .XML file -- nothing works. I usually end up having to create a new machine and point it to an "existing hard disk" -- it works, but brings up an incredibly "old" build (the original, I assume) where I have to redo all the changes I've made over the last year (and wait for updates again!).

I'm on 4.0.8. Host OS is Win7, VM is XP sp3.

VD: error VERR_FILE_NOT_FOUND opening image file 'C:\{path to vdi}'
Result Code: E_FAIL (0X80004005) -- don't you love error messages like this with no way to look them up?

Any help would be appreciated. I *really* don't want to go back to MS's Virtual PC...

thanks!