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and variables like username and password are not being submitted...
The form submits properly for non-AOL users.
During this test if you can't replicate it then you've both narrowed it down to "versions lower than X" and found a solution (upgrade), and if it breaks for you then you can start to debug the process i.e. does it work on other sites? If so what are they doing different?
If memory serves AOL uses a modified version of IE, but in my experience most of their users problems are caused by AOL's own systems, including their "unique" network of load-balancing & caching proxies.
There should be an obvious answer as I doubt even AOL could mess up something as obvious as GET / POST operations.
If all else fails you might want to consider supporting GET requests in addition to POSTs whenever there are not security issues involved (e.g unless you have https-based login screens then GET methods for this would be dangerous for a number of reasons), but for more trivial interactions then the only real disadvanges of GET would be that it leaves unsightly URLs and has a size limit on the length of URL and therefore amount of data.