Need help getting microsoft on-premises gateway work with Azure Analysis services

Question

I am trying out a Microsoft business intelligence solution which consists of a on-prem database/datawarehouse running on a SQL Server 2014 Windows server. On the same sql server is SSIS running a set of SSIS packages stored in the SSIS Catalog. A SQL SSAS Tabular cube is running on the azure portal. Power BI reports are using SSAS live connection.

Well, here is the problem. The customer has not yet an azure portal so I have been testing on free users, but this will be fixed soon. My problem is to get the on-premises datagateway to work with the azure analysis services. I have to mention that the customer already has an on-prem datagateway running on an other machine using a windows service user that they have created.

Well here is what I did. I created access to the datawarehouse for that on-prem windows service user. To create a gateway to the azure portal, I had to ask the db admin person to assist me to create an on-prem gateway on azure from the on-premises data gateway dialog using the registrated account (onmicrosoft.com mail) for that service (not mine). So far so good. He helped me to get into the portal using the onprem service account on azure and there I created a free account and set up analysis services and connected it to the datagateway. I also gave my @onmicrosoft.com account on azure admin rights. Then we logged out of that azure account. From Visual studio 2017 I could now deploy the SSAS tabular using Do not Process. Just before deploying, I correct the data source setting for the SSAS tabular to the IP address like 10.xxx.x.xxx,1433 and set impersonation to Service Account, clicked save and unchecked the encryption and set privacy level to public (will change that later). Then I deployed and that went ok.

BUT I need to process the cube either from SSMS or from a SSIS package. I hade hoped that my @onmicrosoft.com account that has also admin rights on azure could be used. Maybe I have to use the other @onmicrosoft.com account that was created on azure, but I don't have the password to that. I believe this is a credential problem, but I am asking if there is a way around this. On SSMS, I tried running the code below using my Account @onmicrosoft.com account on user name with MTA setting and the asazure://northeurope.asazure.windows.net/xxx on server name

{
  "refresh": {
    "type": "automatic",
    "objects": [
      {
        "database": "My Cube"
      }
    ]
  }
}

It runs some seconds but the it fails with this message

The JSON DDL request failed with the following error: Failed to execute XMLA.
Error returned: 'An error occurred during On-Premise Gateway related activity.
Additional error details:
DM_GWPipeline_Gateway_DataSourceAccessError Received error payload from gateway service with ID 371137: An exception encountered while accessing the target data source.
An exception encountered while accessing the target data source The specified length exceeds maximum capacity of SecureString.
Parameter name: length

Technical Details:
RootActivityId: f4989df9-60c3-445f-8ef7-85fa9f7c48ac
Date (UTC): 3/16/2019 8:00:33 AM
0: PFError::SetLastError() line 2160 + 0x0
(sql\picasso\engine\src\pf\eh\pferror.cpp)
1: PFSetLastError() line 2918 + 0x0 (sql\picasso\engine\src\pf\eh\pferror.cpp)
2: PFSetLastErrorExTag() line 3474 + 0x27
(sql\picasso\engine\src\pf\eh\pferror.cpp)
3: 0x00007FF913041541 (symbolic name unavailable)

I hope somebody can tell me what I am doing wrong or lead me in a correct direction here

Regards