The web installer CSS fails to load, throwing a ServiceDisabledException. I'm not sure what caused this, but the fix is pretty simple. Filing a task for discoverability.
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[406a8f41591f6a029a5472f7] /mw-config/index.php?css=1 Wikimedia\Services\ServiceDisabledException from line 415 of /srv/mw/core/includes/libs/services/ServiceContainer.php: Service disabled: DBLoadBalancer
Backtrace:
#0 /srv/mw/core/includes/MediaWikiServices.php(637): Wikimedia\Services\ServiceContainer->getService(string)
#1 /srv/mw/core/includes/objectcache/ObjectCache.php(181): MediaWiki\MediaWikiServices->getDBLoadBalancer()
#2 /srv/mw/core/vendor/wikimedia/object-factory/src/ObjectFactory.php(172): ObjectCache::{closure}()
#3 /srv/mw/core/includes/objectcache/SqlBagOStuff.php(146): Wikimedia\ObjectFactory::getObjectFromSpec(array)
#4 /srv/mw/core/includes/objectcache/ObjectCache.php(196): SqlBagOStuff->__construct(array)
#5 /srv/mw/core/vendor/wikimedia/object-factory/src/ObjectFactory.php(172): ObjectCache::newFromParams(array)
#6 /srv/mw/core/includes/libs/objectcache/ReplicatedBagOStuff.php(74): Wikimedia\ObjectFactory::getObjectFromSpec(array)
#7 /srv/mw/core/includes/objectcache/ObjectCache.php(196): ReplicatedBagOStuff->__construct(array)
#8 /srv/mw/core/includes/ServiceWiring.php(562): ObjectCache::newFromParams(array, MultiConfig)
#9 /srv/mw/core/includes/libs/services/ServiceContainer.php(451): Wikimedia\Services\ServiceContainer->{closure}(MediaWiki\MediaWikiServices)
#10 /srv/mw/core/includes/libs/services/ServiceContainer.php(419): Wikimedia\Services\ServiceContainer->createService(string)
#11 /srv/mw/core/includes/MediaWikiServices.php(859): Wikimedia\Services\ServiceContainer->getService(string)
#12 /srv/mw/core/includes/ServiceWiring.php(891): MediaWiki\MediaWikiServices->getMainObjectStash()
#13 /srv/mw/core/includes/libs/services/ServiceContainer.php(451): Wikimedia\Services\ServiceContainer->{closure}(MediaWiki\MediaWikiServices)
#14 /srv/mw/core/includes/libs/services/ServiceContainer.php(419): Wikimedia\Services\ServiceContainer->createService(string)
#15 /srv/mw/core/includes/MediaWikiServices.php(1054): Wikimedia\Services\ServiceContainer->getService(string)
#16 /srv/mw/core/includes/installer/WebInstallerOutput.php(139): MediaWiki\MediaWikiServices->getResourceLoader()
#17 /srv/mw/core/includes/installer/WebInstaller.php(1241): WebInstallerOutput->getCSS()
#18 /srv/mw/core/includes/installer/WebInstaller.php(179): WebInstaller->outputCss()
#19 /srv/mw/core/mw-config/index.php(82): WebInstaller->execute(array)
#20 /srv/mw/core/mw-config/index.php(40): wfInstallerMain()
#21 {main}