Inter Company Invoice suddenly stopped working

Installed Apps:
ERPNext: v13.31.1 (HEAD)
Frappe Framework: v13.30.0 (HEAD)

We have successfully been using Inter Company Invoicing for over a year now.

However, yesterday, we encountered the following error:

The error message in the console is as follows:

/api/method/frappe.m…r.make_mapped_doc:1 Failed to load resource: the server responded with a status of 404 ()

jquery.min.js:4 POST 404

send @ jquery.min.js:4
ajax @ jquery.min.js:4 @ request.js:253 @ request.js:103
open_mapped_doc @ create_new.js:373
make_inter_company_invoice @ sales_invoice__js:1150
eval @ sales_invoice__js:989
a @ page.js:576
dispatch @ jquery.min.js:3
r.handle @ jquery.min.js:3

Has something changed and, if so, what do we need to get inter company invoicing working again?

Thanks in advance for any help.