For Test mode runs, the front-end waits up to 60 seconds for a response. If this does not occur, this error is presented, however, an execution continues in the background and can be analyzed in the logs, without a menu panel. (Using filter Test mode).
This can also happen if your computer loses internet connection. In cases of slightly longer executions, the pipeline is still running for up to 5 minutes.
If your pipeline is focused on long runs, take a look at our integration standards document, no items: Data migration and synchronization between systems.