To add some questions into the puzzle, I've been experiencing the WiFi key invalid error since updating my router.
I was using some Netgear as router/wifi access point. Recently I upgraded to TP Link Deco x55 for mesh wifi. When I was using it as *just* the access point the wifi still connected fine.
When I switched to using the x55 as also doing router duties (including local IP reservation) then I started getting the key invalid error.
I run LibreElec on a RPi 3 model B (bought Feb 2018), it's plugged into a powerline adapter for ethernet so I don't consider the wifi functionality essential for my use case.
I also have an older Pi 2 model B v1.1 - when I plug a wifi adapter into the USB, it connects to the same x55 wifi just fine with no complaints.
Both Pi's are on the same build of LibreElec (LibreELEC-RPi2.arm-9.2.8.img). So I wonder if there's a hardware or firmware level compatibility issue rather than it being software/OS.
I tried plugging the wifi dongle into the Pi3 and then realised I don't know how to force it to use USB wifi to see if that would make a difference. Either way, like I say I have it hooked up via ethernet so it's not an essential issue for me but as I found this thread troubleshooting for a resolution I thought I'd pass on my experience. If desired, I can test things if there's info on where to get log information, though I'll be travelling for a week from tomorrow so don't expect anything immediate.
I have a Pi5 on order so when that arrives I'll be interested to see if it works on wifi or the problems continue.