can you click on the dataset in RPT and have the editor come up? If not then RPT can't bind to the port that the dataset uses, which is +1 of the agent port, e.g. the default agent listening port is 7080 so the default dataset port will be 7081
If running on a remote agent location, scale down the schedule and see if the schedule can run OK on local computer.
If so, I suspect a firewall between the remote agent machine blocking port 7081 between the agent machine and RPT workbench machine
simple test is to put in browser http://<RPT host machine>:7081 first on RPT machine to see the expected output which will be a meaningless error message then see if on a browser on the agent machine gets the same results.
If still having an issue please open a Salesforce ticket and we can help you
Thank you
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