All ebook exports are failing with "Error performing an external request", seemingly because https://tools.wmflabs.org/phetools/credits.py is no longer reachable. This might be due to the change to toolforge.org.
A similar error happens for CLI exporting:
./bin/book.php -l en -t Emma
Error: GuzzleHttp\Exception\ClientException: Client error: GET https://tools.wmflabs.org/phetools/credits.py?lang=en&format=json&page=Emma%7CEmma%2FVolume_1%2FChapter_1%7CEmma%2FVolume_1%2FChapter_2[…SNIP…] resulted in a 404 Not Found response
Also reported at https://github.com/wsexport/tool/issues/238