This post, from the Adobe forum thread martbabs linked to, seems to explain the issue:
All solutions, theories or speculations about a fix are a complete waste of time. I do find it entertaining to read some of them, but a total waste of effort on everyone's part. There is no .plist file to delete or preference to tweak. This is an Adobe problem with how Photoshop opens files from a network drive and a problem that came with the latest Creative Suite 5.5 version. I have been using Photoshop for 20 years and have been working directly from an Apple server over a network for the last 10, and NEVER had a problem until I upgraded to Creative Suite 5.5.
Everyone should do themselves a favor and stop trying to come up with a fix. It's Adobe's problem, not Apple's. Apple may have changed the way it communicates from a Mac to a Server but Adobe must adapt to it. Adobe knows that the bulk of Photoshop users are on a Mac, so they need to get over themselves and fix their product.
For those just checking in, here is a horribly un-technical explanation of the problem and a work-around.
Photoshop opens a file into your RAM. The file that is left behind is an empty shell with all but a few bits of data left in it, one of them being the file size and modification stamp. When you or another network user clicks a file, especially in column view, Apple's OS generates a preview. This preview changes the resource data of the empty file that Photoshop leaves behind, and when you attempt to save the file, Photoshop freaks out and provides you with the error and ultimately nukes said empty file from the server, leaving only the RAM version. You can hit save again and re-save the file, but sometimes that isn't done and the file is lost.
The solution, TURN OFF THE PREVIEW COLUMN. By turning off Apple's ability to generate a preview, you are effectively solving the problem. Yes, I admit it's annoying and somewhat difficult to work with, but that's the way it has to be for now. To turn off the preview, go to 'Show View Options' under the 'View' menu and uncheck the 'Show Preview Column' check box. This will disable the preview for the file when in column view, thus preventing other network users or even yourself from creating a preview of an open Photoshop file.
This solution has eliminated all errors for my company, as it's pretty rare to have two people attempt to preview or open the same Photoshop file, so for us, this issue is resolved until Adobe fixes the issue for real.