I loaded the geojson file from here: Victoria, Australia geojson and then immediately tried to export it, and it failed with "Export Failed for an Unknown Reason"
Next I clicked Simplify, selected the defaults, then dropped it to 10%, clicked Repair, then tried Export GeoJson, Export, and it also fails, as above, "Export Failed for an Unknown Reason".
I loaded the geojson file from here: Victoria, Australia geojson and then immediately tried to export it, and it failed with "Export Failed for an Unknown Reason"
Next I clicked Simplify, selected the defaults, then dropped it to 10%, clicked Repair, then tried Export GeoJson, Export, and it also fails, as above, "Export Failed for an Unknown Reason".