That could result from not having appropriate account settings (such as not being an administrator, or your account not having 'Write' privileges), which is a Windows issue. It could also be caused by Windows UAC being a jerk (came as standard with all Win. versions post-Vista. I turned mine completely off).
This may seem like a silly question (but Occam makes me ask it anyway), you're not trying to alter the config file while the game is running, right?
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The only sound that still scares me to this day...
The silence when the pinging stops. 
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