First of all, I would like to say that I have searched and tried everything suggested and still having this problem. I am unable to connect to any wifi available, I have tried my home wifi and my hotspot but KDE Daemon constantly keeps asking for network authentication. My computer has dual-boot with Windows 10 and I have no problems with the wifi on my Windows. I have unchecked the KDE wallet, restarted the network service nothing helped. I have used Kubuntu for the last 6 and everything was fine the first time it happened. I deleted and reinstalled Network manager and made my WiFi open cus I thought maybe the encryptions were the problem still nothing.
As per comments on the original question, your problem differs from mine, so it's pretty unlikely this will help you, but in case it helps anyone else:
I booted into a linux USB, connected to a secured wifi on there, then simply rebooted into my installed version. (Wifi autoconnected as it had been working for me before.)
I have no idea why or how it worked for me, but I saw someone else report that work for them, though retroactively thought it might've been a kernel upgrade. I figured there was little risk or harm in trying the bootable, though, and it helped me.