Are you using the internal wireless of the RPi3 or an external USB dongle? ..and what wireless regulatory domain (country) is your router set to?
Hello,
I am having this problem too (Belgium).
When I select my wifi network (SSID), I punch in my shared secret (strong secret of 32 characters with punctuation marks etc.) and after selecting "done"
I get the message "invalid argument".
(It was like this in libreelec since I got my RPi3 - shortly after it became available...)
When I change my psk to something simple like "aaaabbbb" then I get
a different message: "Network error - Operation aborted"
Thanks for all the support!