Mountain Lion: Not joining preferred WPA2 Wifi network My Mac Mini (2011) has always auto-joined networks worked fine until a few days when he stopped  auto-joining my preferred WPA2 Wifi network when I start it up. When I click Join Other Network and then  Show networks my wifi appears. When I double click it I have to enter my password than my mac joins it. But next time I start my Mac, I have to do that again.

How can I get auto-join to start working again?
Well, now i can answer my question self.

Go to Keychain Access and remove all airport passwords for this network from Login and System keychains.

Then create a new password in System. Enter as password the network password. Item Name and account name: name of the network. Click add. Then double click the new item in list. Change kind to 'airport password'. Then click Access Control and select Allow all applications to access this item.

Restart your computer if it doesn't joins automatically join it manual, next startup it will works fine again.