I've searched your site and noticed a similar thread in 2018 however no resolution and I've started experiencing the same issue recently.
All was working fine until recently, and I haven't made any changes myself.
I have a PlusNet router and a BT wifi extender connected to the router.
Devices connected directly to the PlusNet router via a network cable seem to be fine.
The devices trying to connect via the router wifi seem to get the error, 'Incorrect Password for PlusNet-xxxx'. I have tried to retype the password and this doesn't work. I've shut down the router for several minutes and this doesn't work. I have shut the router down and disconnected all the cables and this hasn't worked. This wasn't happening before.
We are able to connect via the BT wifi extender fine as we have used the same password.