I had a similar problem with Belkin Wemo smart plugs. They didn’t reject a specific letter but some passwords just didn’t work so I had to create a separate SSI

I had a similar problem with Belkin Wemo smart plugs. They didn’t reject a specific letter but some passwords just didn’t work so I had to create a separate SSID and then trial and error a password that would work. Never figured out what the issue was.