Internal error

[0fbdde775914e4a0b6028179] /Special:ViewXML UnexpectedValueException from line 486 of /var/www/html/wiki_new/includes/libs/rdbms/loadbalancer/LoadBalancer.php: Invalid server index index #DB_SLAVE

Backtrace:

#0 /var/www/html/wiki_new/includes/libs/rdbms/loadbalancer/LoadBalancer.php(899): Wikimedia\Rdbms\LoadBalancer->getConnectionIndex()
#1 /var/www/html/wiki_new/includes/libs/rdbms/loadbalancer/LoadBalancer.php(1046): Wikimedia\Rdbms\LoadBalancer->getConnection()
#2 /var/www/html/wiki_new/includes/GlobalFunctions.php(2469): Wikimedia\Rdbms\LoadBalancer->getMaintenanceConnectionRef()
#3 /var/www/html/wiki_new/extensions/DataTransfer/specials/DT_ViewXML.php(23): wfGetDB()
#4 /var/www/html/wiki_new/extensions/DataTransfer/specials/DT_ViewXML.php(240): DTViewXML::getCategoriesList()
#5 /var/www/html/wiki_new/extensions/DataTransfer/specials/DT_ViewXML.php(19): DTViewXML::doSpecialViewXML()
#6 /var/www/html/wiki_new/includes/specialpage/SpecialPage.php(600): DTViewXML->execute()
#7 /var/www/html/wiki_new/includes/specialpage/SpecialPageFactory.php(635): SpecialPage->run()
#8 /var/www/html/wiki_new/includes/MediaWiki.php(307): MediaWiki\SpecialPage\SpecialPageFactory->executePath()
#9 /var/www/html/wiki_new/includes/MediaWiki.php(940): MediaWiki->performRequest()
#10 /var/www/html/wiki_new/includes/MediaWiki.php(543): MediaWiki->main()
#11 /var/www/html/wiki_new/index.php(53): MediaWiki->run()
#12 /var/www/html/wiki_new/index.php(46): wfIndexMain()
#13 {main}