As suggested below, commenting out the if() {} block in UserNameSessionProvider, line 509 made it work again. I'm pasting the XDebug output below:
from /var/www/html/physics/includes/libs/rdbms/database/domain/DatabaseDomain.php(58)
#0 /var/www/html/physics/includes/libs/rdbms/database/domain/DatabaseDomain.php(58): is_string()
#1 /var/www/html/physics/includes/libs/rdbms/database/domain/DatabaseDomain.php(113): Wikimedia\Rdbms\DatabaseDomain->__construct()
#2 /var/www/html/physics/includes/libs/rdbms/database/DBConnRef.php(72): Wikimedia\Rdbms\DatabaseDomain::newFromId()
#3 /var/www/html/physics/includes/libs/rdbms/loadbalancer/LoadBalancer.php(839): Wikimedia\Rdbms\DBConnRef->__construct()
#4 /var/www/html/physics/includes/libs/rdbms/loadbalancer/LoadBalancer.php(755): Wikimedia\Rdbms\LoadBalancer->getConnectionRef()
#5 /var/www/html/physics/includes/libs/rdbms/lbfactory/LBFactory.php(562): Wikimedia\Rdbms\LoadBalancer->getConnection()
#6 /var/www/html/physics/includes/libs/rdbms/lbfactory/LBFactory.php(526): Wikimedia\Rdbms\LBFactory->getMappedDatabase()
#7 /var/www/html/physics/includes/libs/rdbms/DBAccessObjectUtils.php(97): Wikimedia\Rdbms\LBFactory->getPrimaryDatabase()
#8 /var/www/html/physics/includes/user/User.php(400): DBAccessObjectUtils::getDBFromRecency()
#9 /var/www/html/physics/includes/user/User.php(1575): MediaWiki\User\User->load()
#10 /var/www/html/physics/includes/user/User.php(2235): MediaWiki\User\User->getId()
#11 /var/www/html/physics/includes/user/User.php(2243): MediaWiki\User\User->isRegistered()
#12 /var/www/html/physics/includes/session/UserInfo.php(62): MediaWiki\User\User->isAnon()
#13 /var/www/html/physics/includes/session/UserInfo.php(114): MediaWiki\Session\UserInfo->__construct()
#14 /var/www/html/physics/extensions/Auth_remoteuser/src/UserNameSessionProvider.php(322): MediaWiki\Session\UserInfo::newFromName()
#15 /var/www/html/physics/includes/session/SessionManager.php(539): MediaWiki\Extension\Auth_remoteuser\UserNameSessionProvider->provideSessionInfo()
#16 /var/www/html/physics/includes/session/SessionManager.php(245): MediaWiki\Session\SessionManager->getSessionInfoForRequest()
#17 /var/www/html/physics/includes/Request/WebRequest.php(870): MediaWiki\Session\SessionManager->getSessionForRequest()
#18 /var/www/html/physics/includes/Permissions/PermissionManager.php(1594): MediaWiki\Request\WebRequest->getSession()
#19 /var/www/html/physics/includes/Permissions/PermissionManager.php(1536): MediaWiki\Permissions\PermissionManager->getUserPermissions()
#20 /var/www/html/physics/includes/Permissions/UserAuthority.php(285): MediaWiki\Permissions\PermissionManager->userHasRight()
#21 /var/www/html/physics/includes/Permissions/UserAuthority.php(144): MediaWiki\Permissions\UserAuthority->internalAllowed()
#22 /var/www/html/physics/includes/user/User.php(2292): MediaWiki\Permissions\UserAuthority->isAllowed()
#23 /var/www/html/physics/extensions/Auth_remoteuser/src/UserNameSessionProvider.php(509): MediaWiki\User\User->isAllowed()
#24 /var/www/html/physics/includes/session/SessionManager.php(867): MediaWiki\Extension\Auth_remoteuser\UserNameSessionProvider->refreshSessionInfo()
#25 /var/www/html/physics/includes/session/SessionManager.php(558): MediaWiki\Session\SessionManager->loadSessionInfoFromStore()
#26 /var/www/html/physics/includes/session/SessionManager.php(245): MediaWiki\Session\SessionManager->getSessionInfoForRequest()
#27 /var/www/html/physics/includes/Request/WebRequest.php(870): MediaWiki\Session\SessionManager->getSessionForRequest()
#28 /var/www/html/physics/includes/Permissions/PermissionManager.php(1594): MediaWiki\Request\WebRequest->getSession()
#29 /var/www/html/physics/includes/Permissions/PermissionManager.php(1536): MediaWiki\Permissions\PermissionManager->getUserPermissions()
#30 /var/www/html/physics/includes/Permissions/UserAuthority.php(285): MediaWiki\Permissions\PermissionManager->userHasRight()
#31 /var/www/html/physics/includes/Permissions/UserAuthority.php(144): MediaWiki\Permissions\UserAuthority->internalAllowed()
#32 /var/www/html/physics/includes/user/User.php(2292): MediaWiki\Permissions\UserAuthority->isAllowed()
#33 /var/www/html/physics/extensions/Auth_remoteuser/src/UserNameSessionProvider.php(509): MediaWiki\User\User->isAllowed()