Hey there, I took a look at your data and the error is happening because you are trying to add a Plot article that already has a Parent Plot set to it. Because that Plot (0 Level Plots) is not in your campaign already, the system encounters an error and crashes. I will make sure to fix this so the page doesn't lead to a 500 error page and displays a relevant message, but for now and as a workaround, you will need to either remove Back on the Chain Gang's parent and add it on its own, or add the Parent Plot first to the campaign, and then Back on the Chain Gang.

I will leave a message here when the full fix is released, but removing the parent plot from the Plot you tried to add to your campaign should fix the error page in the Plot tab within your Campaign page.