Secure Connection Failed

2 sites - one UK government link and the other a UK bank credit card log in.

I click and I get a failed connection. This has only recently occurred with mojave and again now with catalina.

On firefox this is the message"The page you are trying to view cannot be shown because the authenticity of the received data could not be verified."

On safari it simply says I'm not connected to the internet.

On the credit card login the address bar shows "...GenericError..."

Where can I look in my systems to change anything?

many thanks