Can you please share detail on WLAN type ? Is there any os/device policy that can be causing this ? We would need to do an OTA capture to validate the association and authentication packet between client and AP.
Below is the Client Interoperability from 10.3.1 release note :
Aliffer you can upgrade to latest which is 10.5.x from your current 10.3 version and NO the WLAN config alongwith system config shall be intact.
Below is the image file for latest :
NOTE : Beginning with ZoneDirector 10.5.1, the AP image upgrade uses HTTPS rather than FTP for secure image download. If the AP is trying to download the firmware using FTP, the upgrade will fail. So make sure to keep port 443 enabled between AP and ZD or follow below article to enable legacy upgrade via admin >> upgrade >> enable legacy AP image upgrade.