Sending emails using gmail stopped working (error 401)

My integration between post-smtp and gmail suddently stopped working and it is now returning a 401 (Request is missing required authentication credential). I have reviewed the setup and everything (Client ID, Client Secret, etc) looks correct. This is the error when trying to send a test email:

{
"error": {
"code": 401,
"message": "Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.",
"errors": [
{
"message": "Login Required.",
"domain": "global",
"reason": "required",
"location": "Authorization",
"locationType": "header"
}
],
"status": "UNAUTHENTICATED",
"details": [
{
"@type": "type.googleapis.com/google.rpc.ErrorInfo",
"reason": "CREDENTIALS_MISSING",
"domain": "googleapis.com",
"metadata": {
"service": "gmail.googleapis.com",
"method": "caribou.api.proto.MailboxService.SendMessage"
}
}
]
}
}