It seems you're not the only person to have this problem - it's discussed on the MS forum here: http://answers.microsoft.com/en-us/o...2-e3c4bc33e9f5
The last post (by Tom Ogilvy (MVP)) says:
In the INFO screen where it shows you that your structure is protected and highlighted in yellow, click on the dropdown arrow in the Protect Workbook button to the left of the message. In the dropdown choose/click on Protect Workbook Structure option. This is a toggle based on my testing so if you do that and then go to info again it will not show the structure as being protected.
this is assuming no password was set. It if was, you will need to furnish the password I would guess.
In the dialog that appears in this thread, the Structure option will always be checked - but it is not always in effect.
If that doesn't work then I'm completely out of ideas - sorry!