Apple wireless kboard stops connecting to '09 Mac Pro 5,1

[macmesser]

I have been using an Apple wireless keyboard with my '09 Mac Pro 5,1 (High Sierra) without problem for some time. In the past few days, the keyboard has been losing connection and then failing to connect. I get the input window for the paring ID but the keyboard does not work with it. So keyboard is being found but the keys don't input the code. I'm assuming the keyboard is not the problem as it has not suffered any physical shock and bluetooth also works with other devices. Is there something that might need to be cleared by way of settings/preferences?