I just got a TP-Link TL-WPA4420 powerline kit. Everything is working properly, with a single exception. One of my mac computers is unable to connect via wifi, as I always get the message "incorrect password". The mac is able to connect if I disable the pasword protection in the wireless router. All the other computers connect properly with this password. I checked the router firmware, but it is up to date. My mac computer is an older machine, Macbook Pro 15 inch, Mid 2009, and is running system 10.11.6. I can't upgrade for a newer system.

I would appreciate any help you can give me.