You need to fix your user Autosave directory permission (since you probably upgraded). Unfortunately this is a hidden directory.
1. In the Finder under the "Go" menu select "Go to Folder..."
2. Enter "~/Library/" into the dialog and hit "Go"
3. Highlight the "Autosave Information" directory (This is where your temporary unsaved files are saved)
4. Under the "File" menu select "Get Info"
5. In the Info window expand the "Sharing and Permissions" section at the bottom
6. Give yourself "Read & Write" permission for the Autosave Information directory, and be sure not deny anyone else read and write permissions.
Unfortunately, for some users, this still does not solve their problems. There is a (more painful) alternative as detailed here:
1. Restart Lion, and before you hear the chime, hold down the Command and R keys.
2. You’ll be at the Repair Utilities screen. Click the Utilities item in the Menu Bar, then click Terminal.
3. In the Terminal window, type resetpassword and hit Return.
4. The password reset utility window launches, but you’re not going to reset the password. Instead, click on icon for your Mac’s hard drive at the top. From the dropdown below it, select the user account where you’re having issues.
5. At the bottom of the window, you’ll see an area labeled Reset Home Directory Permissions and ACLs. Click the Reset button there. The reset process takes just a couple of minutes. When it’s done, exit the programs you’ve opened and restart your Mac.
If you are still experiencing problems with permissions, do send us a screenshot of the problem and your Autosave folder.
We are seriously considering disabling Autosave in the next version of the Mac app, as it seems to be creating more problems than it solves. Do let us know if you feel one way or another.