Scratchpads are exported as S-Expressions. But importing scratchpads seems to expect .json.
I'm assuming that the code for importing submissions was rewritten to use json, but this code was also used for importing scratchpads, which should be using S-expressions.
I'm not familiar with this part of code but the relevant functions seem to be: Widgets.file_select_button_named ScratchMode.View.import_button JsUtil.read_file and ScratchMode.Update.update