Test from the swagger then determine the issue is reproducible then check if you are still seeing the error.
A good test would be to perform a "/v2/Assets/{id}/TestConnection" from the Swagger API as the user in question.
You can retrieve the required ID by making a GET request "/v2/Assets " from the Swagger API.
Perform the Authorize using a valid Admin account to determine if the issue is permission related.
The Swagger API can be access through a web browser by navigating to the X0 (Primary network interface) IP address of the appliance follow by service/core/swagger.
e.g https://ip-address/service/core/swagger