What security mode are you using? Preferred security is WPA-Personal. WPA2/AES Only. Some WiFi adapters don't support
AES, so you might want to try
TPIK only or Auto. If some of the devices don't support one of these modes then it could be the DAP logging it when the device attempts to connect and fails.
I recommend that you phone contact D-Link support and ask them about this log entry and see what they say.
Let us know if you find out anything.