Luckily I recognised this as a VE error code, but you should really note the editor or add the project

I don't get this error on loading the page - is it only on save?

I do not speak English, I tried to uderstand what happens with that page. Thanx

Yes The message appears when I want to save.

@Drongou has that error yesterday. He reminds that he was using Youtube at the same time, which may have an impact on Safari(s performance. Page was blocked on error 400, without any possibility of reloading page or going backwards.

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_6) AppleWebKit/601.7.7 (KHTML, like Gecko) Version/9.1.2 Safari/601.7.7

I have tried with only Wikipedia opened on Internet Explorer, Mozzila or Chrome on 3 various PC, and I always have the same message :-(

@AlexMonk-WMF Hi, 9 different users (lots of IP and new users) report the same problem at practically the same time, this afternoon. They explain that the HTTP error 400 Erreur lors du chargement des données depuis le serveur / Error when load data from the server appear when they edits with Visual Editor, but Wikicode editor work.
On this screenshot, Aavitus, a Chrome user, have a javascript alert when he try to open the VE, then redirected to the standard read-mode page.

I have got the same error when try open VE on https://fr.wikipedia.org/wiki/Michel_Juff%C3%A9?veaction=edit
When I type Escape when the alert message is open, another appear with Erreur lors du chargement des données du serveur : nopage: The page parameter must be set. Voulez-vous réessayer ?
I have the problem on Firefox, but this work on IE.

This appears to be frwiki only at this time. Can you try disabling all gadgets?

It's working without any gadget / common.js (my bot account) and after click on Restore to default params on Special:Preferences
But an IP report this too, and he can't have gadget, no ? ...

I saved the list of HTTP queries for https://fr.wikipedia.org/wiki/Michel_Juff%C3%A9?veaction=edit until the first error message is displayed on https://fr.wikipedia.org/w/index.php?title=Utilisateur:Orlodrim/test&oldid=128798674.

The URL that returns HTTP 400 is https://fr.wikipedia.org/api/rest_v1/page/html/undefined/mwTarget?redirect=false.

(I got an error twice in a Firefox session where I was not logged in, but now I cannot reproduce it anymore)

Same as Orlodrim, it still working on different pages after my reconnection for test with my other account. It work too when I'm entirely disconnected (clear cookies).
So, problem with cookies ? Tokens ?

In T141226#2568517, @Orlodrim wrote:

The URL that returns HTTP 400 is https://fr.wikipedia.org/api/rest_v1/page/html/undefined/mwTarget?redirect=false.

Yeah, that must be something going wrong on the client side. I suspect the recent reports might be a different underlying problem with the same user-visible symptom.
This URL would be generated by ArticleTargetLoader if you passed an undefined pageName and the targetName as oldid.

I don't see any way that could happen from the code, though I'm not in a position to be testing/debugging this properly right now. @Esanders?

I'm asking users if they have enabled a particular gadget or script.

Apparently, it is not gadget related, but mostly for people using Chrome.

Do you know if it's been happening on any other wikis?

I'm asking.

Some people managed to have VE back by re-initializing Chrome. That's a little bit too much.

After a new feedback about an article shorter than others, I've been able to test some cases, to isolate a problematic bit of wikitext.

You can trigger an error 400 when you are in a <ref> and you have an internal link into an external link and the external link is contiguous to a template:

<ref>{{en}}[http://articles.chicagotribune.com/1985-09-28/business/8503060172_1_hamish-maxwell-philip-morris-headquarters-in-rye-brook Philip Morris Agrees To Buy General Foods, [[Chicago Tribune]], 28 septembre 1985]</ref> with an internal link (Chicago tribune) into an external link - reproduiced in <ref>{{mdr}}[http://www.perdu.com [[nouveau]] test]</ref> (related article)

Same thing in René-Louis Baron, the first article reported: <ref>Article de presse : [http://realcomposer.fabx.org/Medal%20-%20Inventeur.htm Article de Pierre Dufour - ''Dauphiné libéré'' - Avril [[2000]]].</ref> trigers an error 400.

But <ref>{{vid}}[http://www.youtube.com/watch?v=Zt2IDNi14ZE Laboratoire de recherches MedalMusic sys.''], [[YouTube]].</ref>, in the same article with unbalanced italic doesn't triggers an error 400.

Note: if I paste one of the examples I've found in VE mode of Flow, I have a clear warning concerning parsing : "Unable to transfer content: Error contacting the server for conversion between wikitext and HTML. Please check your Internet connection or try again later if the problem persists. If you still get this error please file a bug."

This bug affects beta editor too which brings me to through this is parsoid isuee not only visualeditor one. Added relevant projects then.

Change 353531 had a related patch set uploaded (by Arlolra; owner: Arlolra):
[mediawiki/services/parsoid@master] T141226: Fix logic when unpacking dom fragments

https://gerrit.wikimedia.org/r/353531

Change 353531 merged by jenkins-bot:
[mediawiki/services/parsoid@master] T141226: Fix logic when unpacking dom fragments

https://gerrit.wikimedia.org/r/353531

Mentioned in SAL (#wikimedia-operations) [2017-05-15T20:20:02Z] <subbu> Updated Parsoid to a182c227 (T141226, T164792, T37247, T153107, T163091, T164006, T161151, T162920, T163549)