Error "element...is not allowed for content model" When Attempting to Open Saved Copies of Published Data Sources

Issue

When opening a downloaded copy of a published data source in Tableau Desktop 2018 or later, you may receive the error:

"error ####, ## element 'xxx' is not allowed for content model"

Additional details about the error message:

This can also happen with published workbooks that are downloaded from Tableau Server.
In the error, the first number refers to the line of the XML for the workbook or data source, the second number refers to the character in the line.
The element name is the tag that is generating the error.

Environment

Tableau Server 2018 and later versions
Downloaded data source or workbook (.tds, .tdsx, .twb, .twbx)

Resolution

Revert to an older version of the workbook/datasource.
1. See if you can find an older version of the file, confirm it opens without error in Tableau Desktop, and republish.
2. Use "Revision History" option on Tableau Server to see if there is an older version of the file that can be downloaded, test in Tableau Desktop, and republish.
If issues persist, submit a case with Tableau Technical Support.

Cause

Between Tableau Server upgrades to 2018 and later versions, if the files have gone through a lot of revisions/republishing it is possible the XML picks up elements from different versions based on how the data is being used (using different sorts, alias changes, etc.), though may retain the version of Tableau Desktop in which it was originally created. If the syntax for the elements, or the elements themselves, do not match the version in the XML, errors may appear.