gateway error when Get data on AWS

Hi

I've installed SQL-server 2017 std with SSIS and SSAS and On-premises gateway on an AWS-hosted server.

the installation seems fine,

- Gateway is configured and is reporting "online - you are good to go"

- Datasource is reporting "connection successful"

- and I've been able to create a report in PowerBI desktop on that instance - and publish it to PowerBI.com,

(I log on to Powerbi.com with other user, but "map user name" back to powerbi@@ad.custdomain.com )

But when trying to load the report I get error "there was a data source access error. Please contact the gateway administrator."

Uhen trying to "get data"/Databases/SQLserverAnalysisServices -> the datasource and gateway presents itself, but when clicking on it i get this message; 

i also checked the On-premises gateways log, to verify that the effective username mappings in Powerbi.com was correct,,, and the username logged on to powerbi.com had been replaced with the one with ssas/server permissions but it seemd as it was trying to replace it once more,, ?.

DM.EnterpriseGateway Information: 0 : 2018-07-12T13:00:16.1217390Z DM.EnterpriseGateway 'info-replaced' 'info-replaced' MGCC 'info-replaced' 1E2E2A20 [DataMovement.PipeLine.GatewayDataAccess] Replace effective user name in adomd connection string from [email protected] to powerbi@@ad.custdomain.com.
DM.EnterpriseGateway Error: 0 : 2018-07-12T13:00:16.4264085Z DM.EnterpriseGateway 'info-replaced' 'info-replaced' MGCC 'info-replaced' BAA79098 [DM.Pipeline.Diagnostics] Exception object created [IsBenign=True]: Microsoft.PowerBI.DataMovement.Pipeline.Diagnostics.AdomdDataAccessErrorResponseException: AdomdException encountered while accessing the target data source.; ErrorShortName: AdomdDataAccessErrorResponseException[ErrorCode=-1056899072,HResult=-2146233088]/Wrapped(AdomdErrorResponseException)[ErrorCode=-1056899072,HResult=-2146233088]

the error-message following is

GatewayPipelineErrorCode=DM_GWPipeline_Gateway_DataSourceAccessError
ErrorCode=-1056899072 --->
[1]Microsoft.PowerBI.DataMovement.Pipeline.Diagnostics.GatewayPipelineWrapperException: Substituted: AdomdErrorResponseException:<pi>Microsoft.AnalysisServices.AdomdClient.AdomdErrorResponseException: The following system error occurred: The security database on the server does not have a computer account for this workstation trust relationship.

The user powerbi@@ad.custdomain.com is used to create and publish the report - so it has access to the data (admin-role  in/on SSAS)

any ideas for troubleshooting woud be apriciated,,;)