r700 APs - 2.4 Ghz clients "Unauthorized"

I have ZD 9.8.3 build 58 and run a WLAN with mostly ZF 7762s that works fine with all clients old and new. Recently I've acquired some R700s and added them to the same WLAN. The R700s seem to only be able to connect 5 GHz clients (newer laptops and phones) - any 2.4 GHz clients trying to connect fail, with ZD showing them as "Unauthorized" even though the passwords are correct. These same clients connect fine to 7762 APs on the same WLAN.

And these are latest generation a-n wifi devices, not old. I've tried a few things like fixing channelization on 2.4 Ghz to 20 but with no luck. Any ideas why this is happening and how to fix it?