To Clarify some points on this issue:
1) This has nothing to do with the 5th mouse button. It just happens that many people use the 5th button to activate the "show desktop" action. The issue happens with ANY button assigned to "show desktop".
2) This is not specific to Logitech mice or people using the Logitech Control Center. For people using LCC, moving the Expose app out of the utilities folder and into the application folder may be a useful workaround, but again, this is only relevant for logitech users using the LCC.
3) Deleting the ~/Library/Preferences/com.apple.symbolichotkeys.plist file may in some cases temporarily solve the problem until a reboot. However, I don't think this is really an issue with this preference file. I saved this file while the machine was in a working state, and also while it was not working and compared the plist files and they are identical.
4) Logging out, and logging back in either as a different user, or as the same user, sometimes solves the problem. For me this is inconsistent.
5) The problem appears even when I create a fresh user. However often logging in and out or switching users temporally solves the problem. However the problem is always there on a cold boot.
6) The only workarounds currently seem to be to use 3rd party software / mouse drivers which allow you to assign these fucntions, or to assign the mouse button to a keyboard keystroke, and assign THAT to the show desktop action.
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Apple, please address this ASAP! Thanks
-N