EN.WP.ST47 wrote:

What request causes the API to return invalid JSON data, and what JSON data does it return?

Maybe the post requests are somehow cut off or it is a specific Wikimedia issue. These errors seem to occur randomly and I have no clue how to reproduce.

One of the failed requests may have looked like:
POST to commons.wikimedia.org/w/api.php

Params:
action: edit
summary: Nominating for deletion
watchlist: preferences
title: File:Copa Airlines Colombia logo.jpg
token: //+
prependtext: {{delete|reason=Per [[:Commons:Deletion requests/File:Copa Airlines.jpg]] ~|subpage=File:Copa Airlines Colombia logo.jpg|year=2012|month=March|day=18}}
format: json

(action=edit&summary=Nominating+for+deletion&watchlist=preferences&title=File%3ACopa+Airlines+Colombia+logo.jpg&token=%2F%2F%2B&prependtext=%7B%7Bdelete%7Creason%3DPer+%5B%5B%3ACommons%3ADeletion+requests%2FFile%3ACopa+Airlines.jpg%5D%5D+~%7Csubpage%3DFile%3ACopa+Airlines+Colombia+logo.jpg%7Cyear%3D2012%7Cmonth%3DMarch%7Cday%3D18%7D%7D&format=json)

I know that the edit-token should be sent last. Will try to get it after the text. And, of course it is desirable having the format=json param first. Will make same changes and return if the issue persists.

(In reply to comment #0)

If you would like to be up-to-date with those issues, just watch
https://secure.wikimedia.org/wikipedia/commons/wiki/MediaWiki_talk:AjaxQuickDelete.js/auto-errors
for new entries with "API request returned code"

Nice! But, you do know you can just change "http" to "https" now, right?

That would give you:
https://commons.wikimedia.org/wiki/MediaWiki_talk:AjaxQuickDelete.js/auto-errors

It's a page you can learn a lot from: E.g. "API request failed (hookaborted):
The modification you tried to make was aborted by an extension hook" is a very
bad error-message. User has to guess which hook of the the thousands.)

See bug #22922 and bug #14261 comment #8 -- if you find others, please report them and mark #14261 for the tracker.

Thanks for the pointers.

After I changed the order in which the object is built, this error seems to persist:

MediaWiki_talk%3AAjaxQuickDelete.js%2Fauto-errors&action=historysubmit&diff=68604416&oldid=68604084

(BTW, the user uses IE8 if this is of importance)

Of course browsers are free how they convert an object into a json-string but this error occurs too often that I would say it is a seldom loss of information somewhere between wikimedia and the client. I could try to improve the error-handler further to get the whole response-text & -headers which is difficult because it should not contain private/secret data (setCookies, tokens, ...).