Use one of the below workaround options.
Host the on-premises database or flat file in the cloud and use a supported connector to recreate the cross-database join. See Keep Data Fresh and Allow Direct Connections to Data Hosted on a Cloud Platform for a list of supported databases.
Publish the connections separately and use data blending at the workbook level, instead of cross-database joins. Fore more information, see Publish a Data Source and Blend Your Data.
This error can also occur on Tableau Server if Web Data Connector is used and when the URL of WDC is not added in the allowlist. Add the URL to allowlist by following the steps in the document below.
Web Data Connectors in Tableau Server
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