Copying from the chat on freenode:

Been trying to debug this one. We found out that if we add

$wgGroupPermissions['*']['autocreateaccount'] = true;

then it works for somebody who has never tried before. However, for somebody who has, and for which it has failed, it still fails.
The logs show

[authentication] Auto-creating Mboisson on login
[queries] ccwiki: SELECT /* User::idFromName 132.203.106.215 */  user_id  FROM `user`   WHERE user_name = 'Mboisson'  LIMIT 1
[authentication] MediaWiki\Auth\AuthManager::autoCreateUser: blacklisted in session p5p6efugdqasbvco026vp7f4jp176v59
[session] SessionBackend "p5p6efugdqasbvco026vp7f4jp176v59" data dirty due to dirty(): AuthManagerSpecialPage->performAuthenticationStep/MediaWiki\Auth\AuthManager->beginAuthentication/MediaWiki\Auth\AuthManager->continueAuthentication/MediaWiki\Session\Session->remove/MediaWiki\Session\SessionBackend->dirty
[session] SessionBackend "p5p6efugdqasbvco026vp7f4jp176v59" save: dataDirty=1 metaDirty=0 forcePersist=0

I confirm that the problem is linked to ticket T151400. Solving ticket T151400 solves this.

I confirm that dropping the triggers does resolve this.

Ok. We'll try this on our dev server and report back on whether it solves the problem or not.

This may or may not be linked to this ticket.
https://phabricator.wikimedia.org/T151400

I believe it is indeed solved.

We do not have the IntraACL addon installed now, but I believe we have installed it in the past. Could it be that it left something behind ?

Ok... I downgraded Babel to the REL1_27 branch now. Running the script gives me a different error this time:
A database query error has occurred.
Query: INSERT IGNORE INTO categorylinks (cl_from,cl_to,cl_sortkey,cl_timestamp,cl_sortkey_prefix,cl_collation,cl_type) VALUES ('141','CC-Cloud','SYSTEM STATUS','20161122214920','','uppercase','page')
Function: LinksUpdate::incrTableUpdate
Error: 1305 PROCEDURE ccwiki.do_insert_category_closure_catlinks does not exist (localhost)

We installed Babel using the latest Language Extension Bundle, which is listed to be compatible with 1.27. https://www.mediawiki.org/wiki/MediaWiki_Language_Extension_Bundle#Latest_release

We run "runJobs.php" every minute in the crontab.

As a comparison, see this page, which uses the same template, but for which it does show:
https://docs.computecanada.ca/mediawiki/index.php?title=Template:Draft&action=edit

Note that we run MediaWiki 1.27.1.