Report refresh failed when appending two sources (On prem(DB2) and cloud date source(Sharepoint))

Hi,

I am appending two sources(On prem(DB2) and cloud date source(Sharepoint)) to produce as a single report.When Published and schedule report refresh, refresh fails with the below error.

Data source error: {"error":{"code":"DM_GWPipeline_Gateway_MashupDataAccessError","pbi.error":{"code":"DM_GWPipeline_Gateway_MashupDataAccessError","parameters":{},"details":[{"code":"DM_ErrorDetailNameCode_UnderlyingErrorCode","detail":{"type":1,"value":"-2147467259"}},{"code":"DM_ErrorDetailNameCode_UnderlyingErrorMessage","detail":{"type":1,"value":"SharePoint: Request failed: The underlying connection was closed: An unexpected error occurred on a send."}},{"code":"DM_ErrorDetailNameCode_UnderlyingHResult","detail":{"type":1,"value":"-2147467259"}},{"code":"Microsoft.Data.Mashup.ValueError.DataSourceKind","detail":{"type":1,"value":"SharePoint"}},{"code":"Microsoft.Data.Mashup.ValueError.DataSourcePath","detail":{"type":1,"value":"https://<orgname>.com/sites/"}},{"code":"Microsoft.Data.Mashup.ValueError.Reason","detail":{"type":1,"value":"DataSource.Error"}}],"exceptionCulprit":1}}} Table: Append1.

Also, under Gateway connection, Cloud connection(Sharepoint connection is pointing to On Premise Gateway connection). 

Thanks in advance for your support.

Thanks