This article is from the Secure Sockets Layer Discussion List FAQ, by Shannon Appel SAppel@consensus.com with numerous contributions by others.
Here are the likely explanations for this:
a) You have downloaded an "export" flavor of the browser with only
40-bit encryption, but your bank requires that you use the "domestic"
flavor of the browser with 128-bit encryption. In this case, you must
download the domestic 128-bit version.
b) Your bank's server keeps a list of the browser versions with which
it will work, and that list has not yet been updated to include the
very latest version(s) of Netscape's browser. In this case, please
ask your bank to add the newest version of Netscape's browser to
their server's list of acceptable versions. Note that many banks will
not accept "Preview Release" versions because they do not contain
domestic (128-bit) encryption.