Are we sure this co-incides with the deployment? Or has someone changed a page onwiki at the same time?

There weren't any changes to the extension at all
https://github.com/wikimedia/mediawiki-extensions-ZeroPortal/compare/wmf/1.29.0-wmf.16...wmf/1.29.0-wmf.17

Similar for it's partners in crime that is ZeroBanner https://github.com/wikimedia/mediawiki-extensions-ZeroBanner/compare/wmf/1.29.0-wmf.16...wmf/1.29.0-wmf.17 and also JsonConfig https://github.com/wikimedia/mediawiki-extensions-JsonConfig/compare/wmf/1.29.0-wmf.16...wmf/1.29.0-wmf.17

Seems a lot like someone changed something on the wiki page, badly... Or a core change

In T161036#3119418, @Reedy wrote:

Seems a lot like someone changed something on the wiki page, badly... Or a core change

That seems to be a recurring theme of some of these "store config on wiki" solutions we've been using. They don't handle bad input well and cause errors much further down that we have zero way to gauge the severity of.

Does anyone outside of Mobile/Zero have an account on zerowiki without creating one via createAndPromote?

Change 344075 had a related patch set uploaded (by Chad):
[mediawiki/extensions/ZeroPortal] Failure to parse json config should result in a usable error

https://gerrit.wikimedia.org/r/344075

Mentioned in SAL (#wikimedia-operations) [2017-03-22T20:05:21Z] <thcipriani@tin> Synchronized php-1.29.0-wmf.17/extensions/ZeroPortal/includes/ApiZeroPortal.php: [[gerrit:344190|Failure to parse json config should result in a usable error]] T161036 (duration: 00m 42s)

In T161036#3119418, @Reedy wrote:

Are we sure this co-incides with the deployment? Or has someone changed a page onwiki at the same time?

I didn't see this (or variations of it) pop up yesterday after re-rolling out to group0

Removing as a train blocker on T160549: MW-1.29.0-wmf.17 deployment blockers, but leaving open if there is more investigation needed.