This issue is caused because Apple's autologin url polling is failing. That url is returning plain html characters, so the OS thinks it didn't connect properly to the Wi-Fi network.
To solve it until Apple fix that polling website, you can change it to this one (it is also official from Apple):
To solve it until Apple fix that polling website, you can change it to this one (it is also official from Apple):
1- Open Terminal
2- Type:
3- Edit the line 8 by changing the url:
4- Press `CONTROL` + `X`, then type `Y`, then `ENTER`. 5- Reboot
This method was originally published by `masklinn` on Reddit.