I'm having the same problem as everyone else here. I'm using Word for Mac 14.2.3 on Lion 10.7.5, with a Word document that is 25 MB and contains several embedded images. I have EndNote installed but not running. I have Dropbox but the file is not located in the Dropbox folder, nor is it in the Google Drive folder. I have Time Machine enabled and don't intend to disable it; it is not active and the backup drive is not attached. Save Word auto-recover info every 10 minutes is enabled. Some time after editing the document (perhaps 10 minutes, ahem) , a pop-up will say that my file name is invalid and it is unable to save the file, and to try saving under a different name. At that time, if I look with Finder in the folder where the file was, it no longer exists, but in its place is a file with a name such as "Word Work File L_1511473207.tmp". Doing a Save-As to the original file name seems to work for a while, until the entire problem repeats.
When this started happening last week (after an upgrade from an earlier version of Office for Mac to this version), I eventually got a message saying the disk had an error and was unwritable. I proceeded to run disk diagnostics, which reported unrecoverable disk errors. The Apple Store ran their diagnostics and decided the disk needed to be replaced. So, after that, I thought the entire problem was the disk drive. But now, the same problem is happening again.
There are two new updates to Word that appear to address a security concern and add something to report error info to Microsoft (perhaps to help diagnose this problem?). I am unable to install these updates; I get the error message "The connection to the update server was interrupted." when I try to install them.
What to do? No one here seems to have a good answer yet.