AJAX HTTP error 403 thrown when exporting

Four buttons are displayed underneath a spreadsheet:
[Save as XLS] [Save as XLSX] [Save as CSV] [Save as PDF]

Clicking any of these buttons will cause a new page to load (its heading: Exporting spreadsheets), and on that new page the following error message will be displayed:

An AJAX HTTP error occurred. HTTP Result Code: 403 Debugging information follows. Path: /batch?id=395&op=do StatusText: Forbidden ResponseText: Access denied | InfoPort @import . . .

(The site's name is InfoPort.)

Above the error message, the page displays "Export process has encountered an error. Please continue to the error page." Clicking the link to "continue to the export page" will load another page (/sheetnode/export/ready), which will download the file as expected.

My guess is that Sheetnode is trying to write to a subdirectory whose access privileges aren't set properly. Can someone tell me which directories are relevant to the Sheetnode module, and what the settings for their ownership and access permissions should be?