RingCentral Sandbox account is not working since today morning.

Same here. Attempting to reset password and returns an error. Unable to reset.

Also, the "Online Account Portal" link in the developer credentials section lands with this:

"Sorry! Please try again later!"

I just check and confirmed that it's down. I am reporting to the team for fixing it.

Yep, ditto here and API calls are returning the below from Postman

"errorCode": "OAU-140",

"message": "Invalid resource owner credentials"

Powershell is just throwing a generic:

Invoke-RestMethod : The underlying connection was closed: An unexpected error occurred on a send.

Service Account portals for dev console and login are showing just no response or the "Sorry! Please try again later!" reponse.

It's back. Please try.