As found on https://wikitech.wikimedia.org/ -- the QR-code generation for 2fa setup with OATHAuth extension seems to be intermittently broken.
From logstash filtering by host:silver:
Expectation (writes <= 0) by MediaWiki::main not met:
query-m: INSERT INTO `oathauth_users` (id,secret,scratch_tokens,is_validated) VALUES ('X') [TRX#xxxxxxxxx]
TransactionProfiler.php line 311 calls wfBacktrace()
TransactionProfiler.php line 200 calls TransactionProfiler->reportExpectationViolated()
Database.php line 1006 calls TransactionProfiler->recordQueryCompletion()
Database.php line 1935 calls DatabaseBase->query()
OATHUser.php line 251 calls DatabaseBase->insert()
SpecialOATH.php line 57 calls OATHUser->enable()
SpecialOATH.php line 38 calls SpecialOATH->enable()
SpecialPage.php line 384 calls SpecialOATH->execute()
SpecialPageFactory.php line 553 calls SpecialPage->run()
MediaWiki.php line 280 calls SpecialPageFactory::executePath()
MediaWiki.php line 704 calls MediaWiki->performRequest()
MediaWiki.php line 506 calls MediaWiki->main()
index.php line 41 calls MediaWiki->run()
index.php line 3 calls require()