The alt-tab did not work for me.
However, I did happen to download the tech data file from within CIMC when it was stuck at "downloading packages" (but before the "pane is dead" error). As soon as the download of the tech data file from CIMC started, the install continued and completed without issue. I don't have another SNS-36XX to replicate this. Maybe the vKVM or browser stops sending the mapped ISO and a new download trigged that process to continue. Not sure, but I'm interested to know what happened there.