In T125938#2005005, @Tgr wrote:

Full error message is Fatal error: Call to undefined method Message::toJson() in /srv/mediawiki/php-1.27.0-wmf.12/extensions/OAuth/frontend/specialpages/SpecialMWOAuthManageConsumers.php on line 337 (and there is no stack trace - @bd808 did something in fatal logging break?)

This is apparently by design - if the user does not have permission, the DAO object returns a message. The idea would be that the user just sees a "this field is private" message without the higher-level code having to worry about access control, but the DAO framework also allows non-string return types via MWOAuthDAO::decodeRow and the two features don't go well together.

Did some grepping and SpecialMWOAuthManageConsumers.php#337 seems to be the only place that can trigger errors currently. Only grants and restrictions are decoded into an object, grants are always visible to a user that passed the special page's own permission check, and this is the only place where restrictions are queried from a consumer wrapped in MWOAuthDAOAccessControl, so it's easy to add a check there. That said, this is a problem with the design and it will be easy to introduce similar bugs in the future.

Change 268859 had a related patch set uploaded (by Gergő Tisza):
Handle error message in SpecialMWOAuthManageConsumers

https://gerrit.wikimedia.org/r/268859

Change 268860 had a related patch set uploaded (by Gergő Tisza):
Fix rights in oauth role

https://gerrit.wikimedia.org/r/268860

Change 268859 merged by jenkins-bot:
Handle error message in SpecialMWOAuthManageConsumers

https://gerrit.wikimedia.org/r/268859

Change 268860 merged by jenkins-bot:
Fix rights in oauth role

https://gerrit.wikimedia.org/r/268860

Change 269333 had a related patch set uploaded (by Gergő Tisza):
Handle error message in SpecialMWOAuthManageConsumers

https://gerrit.wikimedia.org/r/269333

Change 269333 merged by jenkins-bot:
Handle error message in SpecialMWOAuthManageConsumers

https://gerrit.wikimedia.org/r/269333

Backported to production.