Hello Paul,
Seems like there is an issue with the 3rd party tool you are using. Error that you provided us here is generated by that tool and not by us. OAuth2 is something we are still actively working on improving and there might be cases where it does not behave as expected. In order for us to be able to help you with this problem we need more information on what tool you are using and if possible a way for us to repeat. If there is no way to repeat then please provide with detailed description of what is wrong and what exact calls and steps you are doing to repeat it.
eg.: works when parameter-X is in body but doesn't work when parameter-X is in http header
Ideally you would create a bug report with a way to repeat this and we can have a look but good description might be sufficient.
Best Regards,
David D.