I'd be tempted to say if possible see if you can replicate it yourself using whatever the latest version is. Unless they have changed their systems then I wouldn't recommend installing their software on a machine you can't/don't want to do a clean install on.
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.
For what it's worth, I know this works in newer versions of AOL. I have a web app that uses both get and post methods, and cares which one, that works fine in newer versions of AOL. I haven't checked thoroughly to see where the cut-off is, but it might be different for me anyway because my users were having issues with the SSL certificate before they upgraded, too.