The Mediawiki maintenance cron job "generate-fancycaptcha" generates new fancy captchas for the ConfirmEdit extension.
When running this manually for T195392 to ensure it has no issues with PHP 7.2, i noticed it generates the captchas just fine (i even downloaded and looked at 2 random PNGs), but in the next step when it tries to copy the new captchas to storage it fails horribly with unknown error -9999
Generated 10000 captchas in 258.4 seconds Getting a list of old captchas to delete... Done. Copying the new captchas to storage...[Mon Jun 17 01:05:02 2019] .. Warning: Host lookup failed [-9999]: Unknown error -9999 in /srv/mediawiki/php-1.34.0-wmf.8/includes/debug/logger/monolog/LegacyHandler.php on line 218 ... Fatal error: require(/srv/mediawiki/php-1.34.0-wmf.8/includes/json/FormatJson.php): File not found in /srv/mediawiki/php-1.34.0-wmf.8/includes/AutoLoader.php on line 109
Full logs are in /var/log/mediawiki/generate-fancycaptcha on mwmaint1002. This has been going on for a while apparently.