Tried to run it myself in a secured folder and it gave me a different exception.
Open mgcb config in text editor, find /reference: entry and replace all “/” for all “” in the path. Then rename .npl comfig to something other than Content, so it doesn’t run and mess up your mgcb. Just make sure to run it at least once, so that .npl actually generate references and stuff.
Also kinda forgot to mention #begin Content.mgcb. It should… work? It’s treated as a regular resource.