I don't think you are going to be able to. I have the same issue with a Linksys WAG54G and have not seen a solution to it. Something about the router sending incorrect authentication information and Safari being too strict to allow it. A firmware update in the future may solve it.
As I said above I have not seen a way for Safari to set up a Linksys router. Simply use another browser for setup. Personally I use OmniWeb. Given that you touch the router settings very little the time it takes for you to use another browser is nothing more than a slight annoyance.
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