by naming a notebook “heatmap.ipynb”, I triggered an EasyList rule (/heatmap.*?) in uBlock, so a request to the following URL was apparently blocked:/api/contents/heatmap.ipynb?type=notebook

The end result was a nonsensicalError loading Notebook: error, which didn’t improve even after some fixes to error messages in master.

I’m filing this issue without all the misleading information in the original one:#1623