One last thought, can you bring the AP and locate it on the same switch/VLAN as your SZ-100?
In case the WAN link was preventing the AP from getting provisioned, after it seems to have upgraded ok earlier?
If it does join/connect to your SZ-100 when local, then move it back to current network location.
If not... that is one of our first/oldest models of AP, and might need replacing. I hope locating the AP near your SZ-100 helps it get connected.