There are two common causes for this issue. Follow the below steps as appropriate to your situation.
If some users and some workbooks are affected, ensure that either:
The affected users have CONNECT permission to the published data source. See Permission Capabilities and Templates.
OR
The affected workbooks have credentials embedded by the publisher.
If all users and all workbooks or data sources are affected, and a forward proxy server is in use, configure the proxy to ensure the exceptions in Configure for a forward proxy server are specified. Note that this configuration must be done on the primary machine and all worker machines in a distributed environment.
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