I had the same problem using a RPI3 and this is what i did to fix it.
It was not the password or the chipset in my opinion, Also it was in Libreelec 7.0.2.
I had to reset to the defaults by going to System/Internet Access and the option at the bottom of the page is Reset above settings to default.
After the reset went to libreelec and connections and was able to put the password in.
Hope this helps
FYI i am very new to the RPI and different systems you can run with it.
