Azure SQL mirroring to Fabric fails with Error occured while interacting with landing zone

Hi spaceman127 
Thanks for your time,

I removed some debug code to make it more compact, but in essence this is my pyspark notebook.
I did not include the connection object, this was created in GUI and I refer to the same connection object both in my GUI test and API test.
When doing the calls I authenticate as myself. The connection object is using service principal towards the database.

import requests, json, base64, uuid
 
token = mssparkutils.credentials.getToken("pbi")

workspaceId = "zzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzzz"
connectionId = "xxxxxxxxxxxxxxxxxxxxxxxxx"
DATABASE = "mirrortesting"

# ====== HEADERS ======
headers = {
    "Authorization": f"Bearer {token}",
    "Content-Type": "application/json"
}

# ====== Create Mirroring Variables ======
 
urlCreateMirror = "https://api.fabric.microsoft.com/v1/workspaces/" + workspaceId + "/mirroredDatabases"

payloadUnencoded = {
    "properties": {
        "source": {
            "type": "AzureSqlDatabase",
            "typeProperties": {
                "connection": connectionId,
                 "database": DATABASE
            }
        },
        "target": {
            "type": "MountedRelationalDatabase",
            "typeProperties": {
                "defaultSchema": "dbo",
                "format": "Delta",
                 "landingZone": {
                 "type": "Managed"
                 }
            }
        }
    }
}
 
b64 = base64.b64encode(json.dumps(payloadUnencoded).encode('utf-8')).decode('utf-8')
display_name = f"Mirror Instance {DATABASE}"
 
body = {
    "displayName": display_name,
    "description": "Mirror created in notebook",
    "definition": {
        "parts": [
            {
                "path": "mirroring.json",
                "payload": b64,
                "payloadType": "InlineBase64"
            }
        ]
    }
}


# ====== Create Mirroring API CALL =====
resp = requests.post(urlCreateMirror, headers=headers, data=json.dumps(body), timeout=120)
mirroredDbId = resp.json()["id"]
time.sleep(20) #wait for Initialized



# ====== Start Mirroring ======
urlStart = "https://api.fabric.microsoft.com/v1/workspaces/" + workspaceId + "/mirroredDatabases/" + mirroredDbId + "/startMirroring"
resp = requests.post(urlStart, headers=headers, data=json.dumps({}), timeout=120)
time.sleep(20) #wait for 'Running'



# ====== Check status ======
urlStatus = "https://api.fabric.microsoft.com/v1/workspaces/" + workspaceId + "/mirroredDatabases/" + mirroredDbId + "/getMirroringStatus"
respStatus = requests.post(urlStatus, headers=headers, data=json.dumps({}), timeout=120)
print(json.dumps(respStatus.json(), indent=2))