So far on wikis bgwiki, enwikiquote, nowiki during the stub (metadata) dumps. A sample stack trace:
```
[20221101142049]: InvalidArgumentException from line 2322 of /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadbalancer/LoadBalancer.php: No server with index '4'
#0 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadbalancer/LoadBalancer.php(1191): Wikimedia\Rdbms\LoadBalancer->getServerInfoStrict(4)
#1 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadbalancer/LoadBalancer.php(1124): Wikimedia\Rdbms\LoadBalancer->reallyOpenConnection(4, Object(Wikimedia\Rdbms\DatabaseDomain), Array)
#2 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadbalancer/LoadBalancer.php(957): Wikimedia\Rdbms\LoadBalancer->reuseOrOpenConnectionForNewRef(4, Object(Wikimedia\Rdbms\DatabaseDomain), 3)
#3 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadmonitor/LoadMonitor.php(237): Wikimedia\Rdbms\LoadBalancer->getServerConnection(4, '', 3)
#4 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadmonitor/LoadMonitor.php(172): Wikimedia\Rdbms\LoadMonitor->computeServerStates(Array, Array)
#5 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/objectcache/wancache/WANObjectCache.php(1688): Wikimedia\Rdbms\LoadMonitor->Wikimedia\Rdbms\{closure}(Array, 1, Array, 1667312380.4293, Array)
#6 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/objectcache/wancache/WANObjectCache.php(1521): WANObjectCache->fetchOrRegenerate('global:rdbms-se...', 1, Object(Closure), Array, Array)
#7 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadmonitor/LoadMonitor.php(181): WANObjectCache->getWithSetCallback('global:rdbms-se...', 1, Object(Closure), Array)
#8 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadmonitor/LoadMonitor.php(104): Wikimedia\Rdbms\LoadMonitor->getServerStates(Array)
#9 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadbalancer/LoadBalancer.php(553): Wikimedia\Rdbms\LoadMonitor->scaleLoads(Array)
#10 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadbalancer/LoadBalancer.php(509): Wikimedia\Rdbms\LoadBalancer->getReaderIndex('dump', 'bgwiki')
#11 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadbalancer/LoadBalancer.php(930): Wikimedia\Rdbms\LoadBalancer->getConnectionIndex(-1, Array, 'bgwiki')
#12 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/database/DBConnRef.php(103): Wikimedia\Rdbms\LoadBalancer->getConnectionInternal(-1, Array, 'bgwiki', 0)
#13 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/database/DBConnRef.php(117): Wikimedia\Rdbms\DBConnRef->ensureConnection()
#14 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/database/DBConnRef.php(356): Wikimedia\Rdbms\DBConnRef->__call('selectRow', Array)
#15 /srv/mediawiki/php-1.40.0-wmf.7/includes/page/WikiPage.php(408): Wikimedia\Rdbms\DBConnRef->selectRow(Array, Array, Array, 'WikiPage::pageD...', Array, Array)
#16 /srv/mediawiki/php-1.40.0-wmf.7/includes/page/WikiPage.php(430): WikiPage->pageData(Object(Wikimedia\Rdbms\DBConnRef), Array, Array)
#17 /srv/mediawiki/php-1.40.0-wmf.7/includes/page/WikiPage.php(470): WikiPage->pageDataFromTitle(Object(Wikimedia\Rdbms\DBConnRef), Object(Title), Array)
#18 /srv/mediawiki/php-1.40.0-wmf.7/includes/page/WikiPage.php(577): WikiPage->loadPageData()
#19 /srv/mediawiki/php-1.40.0-wmf.7/includes/export/XmlDumpWriter.php(259): WikiPage->getId()
#20 /srv/mediawiki/php-1.40.0-wmf.7/includes/export/WikiExporter.php(549): XmlDumpWriter->openPage(Object(stdClass))
#21 /srv/mediawiki/php-1.40.0-wmf.7/includes/export/WikiExporter.php(491): WikiExporter->outputPageStreamBatch(Object(Wikimedia\Rdbms\MysqliResultWrapper), Object(stdClass))
#22 /srv/mediawiki/php-1.40.0-wmf.7/includes/export/WikiExporter.php(315): WikiExporter->dumpPages('page_id >= 5726...', false)
#23 /srv/mediawiki/php-1.40.0-wmf.7/includes/export/WikiExporter.php(198): WikiExporter->dumpFrom('page_id >= 5726...', false)
#24 /srv/mediawiki/php-1.40.0-wmf.7/maintenance/includes/BackupDumper.php(359): WikiExporter->pagesByRange(57260, 59115, false)
#25 /srv/mediawiki/php-1.40.0-wmf.7/maintenance/dumpBackup.php(82): BackupDumper->dump(1, 1)
#26 /srv/mediawiki/php-1.40.0-wmf.7/maintenance/includes/MaintenanceRunner.php(309): DumpBackup->execute()
#27 /srv/mediawiki/php-1.40.0-wmf.7/maintenance/doMaintenance.php(85): MediaWiki\Maintenance\MaintenanceRunner->run()
#28 /srv/mediawiki/php-1.40.0-wmf.7/maintenance/dumpBackup.php(144): require_once('/srv/mediawiki/...')
#29 /srv/mediawiki/multiversion/MWScript.php(120): require_once('/srv/mediawiki/...')
#30 {main}
Wikimedia\Rdbms\DBConnectionError from line 1359 of /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadbalancer/LoadBalancer.php: Cannot access the database: No working replica DB server: Unknown error
#0 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadbalancer/LoadBalancer.php(522): Wikimedia\Rdbms\LoadBalancer->reportConnectionError()
#1 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/loadbalancer/LoadBalancer.php(930): Wikimedia\Rdbms\LoadBalancer->getConnectionIndex(-1, Array, 'bgwiki')
#2 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/database/DBConnRef.php(103): Wikimedia\Rdbms\LoadBalancer->getConnectionInternal(-1, Array, 'bgwiki', 0)
#3 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/database/DBConnRef.php(117): Wikimedia\Rdbms\DBConnRef->ensureConnection()
#4 /srv/mediawiki/php-1.40.0-wmf.7/includes/libs/rdbms/database/DBConnRef.php(356): Wikimedia\Rdbms\DBConnRef->__call('selectRow', Array)
#5 /srv/mediawiki/php-1.40.0-wmf.7/includes/page/WikiPage.php(408): Wikimedia\Rdbms\DBConnRef->selectRow(Array, Array, Array, 'WikiPage::pageD...', Array, Array)
#6 /srv/mediawiki/php-1.40.0-wmf.7/includes/page/WikiPage.php(430): WikiPage->pageData(Object(Wikimedia\Rdbms\DBConnRef), Array, Array)
#7 /srv/mediawiki/php-1.40.0-wmf.7/includes/page/WikiPage.php(470): WikiPage->pageDataFromTitle(Object(Wikimedia\Rdbms\DBConnRef), Object(Title), Array)
#8 /srv/mediawiki/php-1.40.0-wmf.7/includes/page/WikiPage.php(577): WikiPage->loadPageData()
#9 /srv/mediawiki/php-1.40.0-wmf.7/includes/export/XmlDumpWriter.php(259): WikiPage->getId()
#10 /srv/mediawiki/php-1.40.0-wmf.7/includes/export/WikiExporter.php(549): XmlDumpWriter->openPage(Object(stdClass))
#11 /srv/mediawiki/php-1.40.0-wmf.7/includes/export/WikiExporter.php(491): WikiExporter->outputPageStreamBatch(Object(Wikimedia\Rdbms\MysqliResultWrapper), Object(stdClass))
#12 /srv/mediawiki/php-1.40.0-wmf.7/includes/export/WikiExporter.php(315): WikiExporter->dumpPages('page_id >= 6446...', false)
#13 /srv/mediawiki/php-1.40.0-wmf.7/includes/export/WikiExporter.php(198): WikiExporter->dumpFrom('page_id >= 6446...', false)
#14 /srv/mediawiki/php-1.40.0-wmf.7/maintenance/includes/BackupDumper.php(359): WikiExporter->pagesByRange(64468, 66244, false)
#15 /srv/mediawiki/php-1.40.0-wmf.7/maintenance/dumpBackup.php(82): BackupDumper->dump(1, 1)
#16 /srv/mediawiki/php-1.40.0-wmf.7/maintenance/includes/MaintenanceRunner.php(309): DumpBackup->execute()
#17 /srv/mediawiki/php-1.40.0-wmf.7/maintenance/doMaintenance.php(85): MediaWiki\Maintenance\MaintenanceRunner->run()
#18 /srv/mediawiki/php-1.40.0-wmf.7/maintenance/dumpBackup.php(144): require_once('/srv/mediawiki/...')
#19 /srv/mediawiki/multiversion/MWScript.php(120): require_once('/srv/mediawiki/...')
#20 {main}
```
I guess this might be the result of the patch merge from T298485 so I'm adding the DBA project preemptively, feel free to move it if that's wrong.