It could be that there’s a problem with the file that stores the passwords (wpa_supplicant.conf). Moving it (so the system creates a new one) sometimes solves these issues. You’ll need adb for a root-enabled file manager to do this, see this post for details. Note that moving/deleting the file will cause the system to forget previously entered Wifi passwords. Alternatively, some wifi password managers may also correct any issues with this file.
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