Hi @matthew.babbage,
Welcome to the Auth0 Community!
I understand you encountered issues with the Management API and the API Explorer pages on the Auth0 Dashboard.
Our API Explorer page on dashboard throws "Error! API Error. Please contact support if the problem persists" FAQ states that you can expect this behavior to happen whenever there is an empty or invalid action trigger code, preventing the client credentials flow. In this case, we must fix or remove the Action trigger from the flow.
In my investigation, I found your FusionOSAddUser Action empty. Once that is fixed/removed from the flow, this error should go away.
Please let me know if there is anything else I can do to help.
Thanks,
Rueben