400 Bad Request when attempting to add devices

[burtonjk]

I am getting frequent failures when adding devices through the SmartApp with Failure: 400 Bad Request. It has worked but I have never been able to work out what the circumstances are for success/fail

My SmartApp looks good (Deployed to test) in Smartthings Developer
My SmartApp is stil accessibile at https://my-url/smartthings/smartapp
Devices that I have managed to add via the SmartApp continue to work fine

Hoping someone can advise - happy to provide more info as required.

Node-Red 3.0.2
node-red-contrib-smartthings 0.0.43
iOS Smartthings App