From https://gerrit.wikimedia.org/r/c/mediawiki/extensions/GrowthExperiments/+/716947 :
22:20:09 mw-error.log:2021-09-06 05:13:04 266edf23ca17 wikidb: [6685c50fca073b5a6aabf2bc] /api.php?format=json MWException from line 113 of /workspace/src/extensions/GrowthExperiments/includes/Mentor.php: Homepage Mentorship module: no mentor available for user NewUser-0.9135686845886051-Iñtërnâtiônàlizætiøn 22:20:09 mw-error.log:#0 /workspace/src/extensions/GrowthExperiments/includes/Mentor.php(44): GrowthExperiments\Mentor::selectMentor(User) 22:20:09 mw-error.log:#1 /workspace/src/extensions/GrowthExperiments/includes/HomepageHooks.php(423): GrowthExperiments\Mentor::newFromMentee(User, boolean) 22:20:09 mw-error.log:#2 /workspace/src/includes/HookContainer/HookContainer.php(329): GrowthExperiments\HomepageHooks::onLocalUserCreated(User, boolean) 22:20:09 mw-error.log:#3 /workspace/src/includes/HookContainer/HookContainer.php(132): MediaWiki\HookContainer\HookContainer->callLegacyHook(string, array, array, array) 22:20:09 mw-error.log:#4 /workspace/src/includes/HookContainer/HookRunner.php(2432): MediaWiki\HookContainer\HookContainer->run(string, array) 22:20:09 mw-error.log:#5 /workspace/src/includes/auth/AuthManager.php(1446): MediaWiki\HookContainer\HookRunner->onLocalUserCreated(User, boolean) 22:20:09 mw-error.log:#6 /workspace/src/includes/auth/AuthManager.php(1163): MediaWiki\Auth\AuthManager->continueAccountCreation(array) 22:20:09 mw-error.log:#7 /workspace/src/includes/api/ApiAMCreateAccount.php(94): MediaWiki\Auth\AuthManager->beginAccountCreation(User, array, string) 22:20:09 mw-error.log:#8 /workspace/src/includes/api/ApiMain.php(1593): ApiAMCreateAccount->execute() 22:20:09 mw-error.log:#9 /workspace/src/includes/api/ApiMain.php(529): ApiMain->executeAction() 22:20:09 mw-error.log:#10 /workspace/src/includes/api/ApiMain.php(500): ApiMain->executeActionWithErrorHandling() 22:20:09 mw-error.log:#11 /workspace/src/api.php(90): ApiMain->execute() 22:20:09 mw-error.log:#12 /workspace/src/api.php(45): wfApiMain() 22:20:09 mw-error.log:#13 {main}