We have a customer running 20x R700 and therefore running firmware 9.8 on the ZD.
Generally it is working great except for iPads. They can connect to the network and work but after 5mins connections is lost. The iPad still shows as connected but there is no internet, airplay or anything network related.
If we disable and re-enable wifi then it works again for another 15ish minutes.
Please open a support case. 9.8 is just about to "freeze" for the GA release and if this is a new bug we'd like to identify it in time to resolve.
That said, it does look similar to an issue already corrected in the 9.8 GA. If these are first/second generation iPads, it's probably the same issue. Setting encryption to WPA would also be a valid test.