'encrypt iphone backup' greyed out / Cant't disable checkmark

Hello,

Since a 3/4 of a year I have this problem. It has yet been discussed here:

http://discussions.apple.com/thread.jspa?messageID=10511017

But the issue isn't solved. Is there a solution for mac? I really want to get rid of this.

OSX 10.6.3
iTunes 9.1.1
iPhone 2G 3.1.2
MacBook Pro 15" 2010