pytorch_smdataparallel_maskrcnn_demo aborts unexpectedly

https://github.com/aws/amazon-sagemaker-examples/blob/67925dfb813031402deb65150019f5b6d601e0b5/training/distributed_training/pytorch/data_parallel/maskrcnn/pytorch_smdataparallel_maskrcnn_demo.ipynb#L168

In cell 6 the notebook stops (even with repeated attempts) after 491 output lines with the following message: "File Save Error for pytorch_smdataparallel_maskrcnn_demo.ipynb Invalid response: 504 Gateway Timeout" (In an earlier attempt the second line was "NetworkError when attempting to fetch resource.") When I close the tab and want to restart the Jupyter Lab from the overview of the notebook instances, only a white page appears.