Cause
The body does not meet the contract of the operation. The response may include an issues member listing the problems found. At the token endpoint, the request mixes two client authentication methods.
What to do
Fix the fields listed in issues or use a single client authentication method.
application/problem+json
{
"type": "https://masnet.com/developers/errors/invalid_request",
"title": "Invalid request",
"status": 400,
"detail": "...",
"code": "invalid_request",
"correlation_id": "..."
}