I am aware this reply is over a year later but I am assuming that there may be others this issue is happening to.
I also had the identical problem. When SSID was set to broadcast, the One Plus and Samsung Galaxy S5 would connect without any issues as normal. When the SSID was set not to broadcast, I would have an authentication error and/or show the WiFi connection saved. All this started after the router's f/w and both phones' (Android)OS were updated. This was on both the 2.4GHz and 5GHz frequencies.
Here's the solution - this is done on the phones/mobile units having the connection issues AND having your router's SSID's set not to broadcast.
1) Hold and press the WiFi connection and select "Forget"
You will now more than likely see those available. Make sure you don't use these connections
2) You now have to chose the option to "Add Network" from the menu (One Plus) or "Add Wifi Network" at the bottom of the WiFi settings on the Galaxy S5.
3) Manually type in the SSID you want to connect to, select the type of security and enter the password for your WiFi
4) Repeat steps 2) and 3) if you have a second SSID you want to connect to
5) You should be able to now connect to your Wifi network, if not reboot your phone/mobile device and you should now be able to connect.
Hope this helps you out.