Buildx push fails on Oracle Cloud registry

Question

I'm trying to build and push with buildx an image on Oracle Cloud registry, but it fails with the following error:

failed to solve: rpc error: code = Unknown desc = fra.ocir.io/***: failed to authorize: failed to fetch oauth token: unexpected status from GET request to https://fra.ocir.io/*** 400 Bad Request

This is the buildx command:

docker buildx build --platform linux/amd64,linux/arm64,linux/arm/v7 -t fra.ocir.io/*** --push .

This is my config.json that works for any other simple push without using buildx:

{
    "auths": {
            "fra.ocir.io": {
                    "auth": "***"
            }
    }
}

It fails even if before I do docker login on Oracle cloud. NOTE: It works fine if I run the command above pushing on docker hub.

Here some details about the version:

Client: Docker Engine - Community
 Version:           19.03.5
 API version:       1.40
 Go version:        go1.12.12
 Git commit:        633a0ea
 Built:             Wed Nov 13 07:25:41 2019
 OS/Arch:           linux/amd64
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.6
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.13.15
  Git commit:       8728dd2
  Built:            Fri Apr  9 22:43:57 2021
  OS/Arch:          linux/amd64
  Experimental:     true
 containerd:
  Version:          1.4.4
  GitCommit:        05f951a3781f4f2c1911b05e61c160e9c30eaa8e
 runc:
  Version:          1.0.0-rc93
  GitCommit:        12644e614e25b05da6fd08a38ffa0cfe1903fdec
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0