Also I am getting same error with 3-legged authorization flow(Server side web app) settings. created ticket as well but no response from them.

What is the ticket number?

Is this affected a certain user or all users?

BrightGaugeSoftware : Asking the support team to take a look into the ticket : 13889695

@Mohan , Have you opened Dev Support ticket ?

After checking with the engineers. I learn that recently we reenforce OAuth with the client id and client secret in the auth header. So if your current auth code does not include the client secret, it will fail with the error codeOAU-123 as you reported.
https://developers.ringcentral.com/api-reference/Authorization

For confidential clients (server applications) client_id is not enough, you need to provide Authorization header (containing Base64-encoded client_id and client_secret) or client_assertion parameter via JWT client request.

Please update your oauth header and try again.

Let me know if this helps.