Open the working report up in SSRS, go to properties and click the "Edit" button, this will allow you to download the RDL file.
Go to the same report that's broken, go to the properties and click the "Update" button, and then upload the downloaded RDL file, and be sure to select overwrite option, click OK or Apply then do to the data source and verify that you have the correct one.
Execute the report in the Report Manager.
Does this resolve the issue?
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