So this line is bad:

data = json.loads(request.get("BODY") or "{}")

With Zope 5.8.4+ this fails when the request BODY is larger than 1MB.
See discussion in plone/Products.CMFPlone#3848 and zopefoundation/Zope#1142.
See my summary of the history and the steps taken so far.

Temporarily fixed in #1729 by disabling a Zope form memory limit check.