始めて投稿させていただきます。以下の現象が発生し、困っています。過去の投稿を探してみましたが、同様の事象にはたどり着けませんでした。お知恵を拝借できましたら幸甚です。

[EC-CUBE] ４系
[レンタルサーバ] XREA
[PHP] php73
[導入プラグインの有無] ReceiptPdf2, MypageReceipt2
[現象]プラグインの動作確認のため、プラグインを一旦無効にしたところ、エラーが発生し、その後、オンラインショップにも管理画面にもアクセスできなくなってしまいました。
オンラインショップURL：https://motherearthresearchinstitute.org/shop/

エラー内容は以下の通りです。どうぞよろしくお願いいたします。

(1/1) FatalThrowableError
Function name must be a string

in atomlib.php line 641
at x('/virtual/anju/public_html/motherearthresearchinstitute.org/shop/.devcontainer')
in atomlib.php line 511
at include('/virtual/anju/public_html/motherearthresearchinstitute.org/shop/src/Eccube/Request/atomlib.php')
in DebugClassLoader.php line 155
at DebugClassLoader->loadClass('Eccube\\Request\\atomlib')
at spl_autoload_call('Eccube\\Request\\atomlib')
at class_exists('Eccube\\Request\\atomlib')
in ClassExistenceResource.php line 82
at ClassExistenceResource->isFresh(0)
in ContainerBuilder.php line 371
at ContainerBuilder->getReflectionClass('Eccube\\Request\\atomlib')
in FileLoader.php line 150
at FileLoader->findClasses('Eccube\\', '../../../src/Eccube/*', '../../../src/Eccube/{Annotation,Common,Entity,Exception,Log,Plugin,ServiceProvider,Resource,Doctrine/ORM/tools/}')
in FileLoader.php line 57
at FileLoader->registerClasses(object(Definition), 'Eccube\\', '../../../src/Eccube/*', '../../../src/Eccube/{Annotation,Common,Entity,Exception,Log,Plugin,ServiceProvider,Resource,Doctrine/ORM/tools/}')
in YamlFileLoader.php line 578
at YamlFileLoader->parseDefinition('Eccube\\', array('resource' => '../../../src/Eccube/*', 'exclude' => '../../../src/Eccube/{Annotation,Common,Entity,Exception,Log,Plugin,ServiceProvider,Resource,Doctrine/ORM/tools/}'), '/virtual/anju/public_html/motherearthresearchinstitute.org/shop/app/config/eccube/services.yaml', array('autowire' => true, 'autoconfigure' => true, 'public' => false, 'bind' => array('$cartPurchaseFlow' => object(BoundArgument), '$shoppingPurchaseFlow' => object(BoundArgument), '$orderPurchaseFlow' => object(BoundArgument), '$_orderStateMachine' => object(BoundArgument))))
in YamlFileLoader.php line 237
at YamlFileLoader->parseDefinitions(array('parameters' => array('container.dumper.inline_class_loader' => true, 'env(ECCUBE_LOCALE)' => 'ja', 'env(ECCUBE_TIMEZONE)' => 'Asia/Tokyo', 'env(ECCUBE_CURRENCY)' => 'JPY', 'locale' => '%env(ECCUBE_LOCALE)%', 'timezone' => '%env(ECCUBE_TIMEZONE)%', 'currency' => '%env(ECCUBE_CURRENCY)%'), 'services' => array('Eccube\' => array('resource' => '../../../src/Eccube/*', 'exclude' => '../../../src/Eccube/{Annotation,Common,Entity,Exception,Log,Plugin,ServiceProvider,Resource,Doctrine/ORM/tools/}'), 'Eccube\Common\EccubeConfig' => array('public' => true), 'Eccube\Service\TaxRuleService' => array('lazy' => true, 'public' => true), 'Eccube\Repository\TaxRuleRepository' => array('lazy' => true), 'Eccube\Service\CartService' => array('lazy' => true), 'Eccube\Service\SystemService' => array('lazy' => true, 'public' => true), 'Eccube\Service\Composer\ComposerProcessService' => array('lazy' => true), 'Eccube\Service\Composer\ComposerServiceInterface' => array('factory' => array('Eccube\\Service\\Composer\\ComposerServiceFactory', 'createService')), 'Eccube\Controller\' => array('resource' => '../../../src/Eccube/Controller', 'tags' => array('controller.service_arguments')), 'Eccube\Twig\Extension\EccubeBlockExtension' => array('tags' => array('twig.extension'), 'bind' => array('$blockTemplates' => '%eccube_twig_block_templates%')), 'Plugin\' => array('resource' => '../../../app/Plugin/*', 'exclude' => '../../../app/Plugin/*/{Entity,Resource,ServiceProvider,Tests,Codeception,DoctrineMigrations}'), 'Customize\' => array('resource' => '../../../app/Customize/*', 'exclude' => '../../../app/Customize/{Entity,Resource,Tests}'), 'Customize\Controller\' => array('resource' => '../../../app/Customize/Controller', 'tags' => array('controller.service_arguments')), 'twig.extension.stringloader' => array('class' => 'Twig_Extension_StringLoader', 'tags' => array('twig.extension')), 'eccube.collector.core' => array('class' => 'Eccube\\DataCollector\\EccubeDataCollector', 'tags' => array(array('name' => 'data_collector', 'template' => '@toolbar/eccube.html.twig', 'id' => 'eccube_core', 'priority' => -512))), 'eccube.security.success_handler' => array('class' => 'Eccube\\Security\\Http\\Authentication\\EccubeAuthenticationSuccessHandler'), 'eccube.security.failure_handler' => array('class' => 'Eccube\\Security\\Http\\Authentication\\EccubeAuthenticationFailureHandler'), 'eccube.logger' => array('class' => 'Eccube\\Log\\Logger', 'arguments' => array('@Eccube\\Request\\Context', '@monolog.logger', '@monolog.logger.front', '@monolog.logger.admin'), 'lazy' => true, 'public' => true), 'eccube.log.formatter.line' => array('class' => 'Monolog\\Formatter\\LineFormatter', 'arguments' => array('[%%datetime%%] %%channel%%.%%level_name%% [%%extra.session_id%%] [%%extra.uid%%] [%%extra.user_id%%] [%%extra.class%%:%%extra.function%%:%%extra.line%%] - %%message%% %%context%% [%%extra.http_method%%, %%extra.url%%, %%extra.ip%%, %%extra.referrer%%, %%extra.user_agent%%]')), 'Eccube\Log\Processor\SessionProcessor' => array('tags' => array(array('name' => 'monolog.processor'))), 'Eccube\Log\Processor\TokenProcessor' => array('tags' => array(array('name' => 'monolog.processor'))), 'Monolog\Processor\UidProcessor' => array('tags' => array(array('name' => 'monolog.processor'))), 'Monolog\Processor\IntrospectionProcessor' => array('arguments' => array('!php/const:Monolog\\Logger::DEBUG', array('Eccube\\\\Log', 'Psr\\\\Log')), 'tags' => array(array('name' => 'monolog.processor'))), 'Symfony\Bridge\Monolog\Processor\WebProcessor' => array('arguments' => array(array('url' => 'REQUEST_URI', 'ip' => 'REMOTE_ADDR', 'http_method' => 'REQUEST_METHOD', 'server' => 'SERVER_NAME', 'referrer' => 'HTTP_REFERER', 'user_agent' => 'HTTP_USER_AGENT')), 'tags' => array(array('name' => 'monolog.processor'), array('name' => 'kernel.event_listener', 'event' => 'kernel.request', 'priority' => 1024))), 'Symfony\Component\HttpFoundation\ParameterBag' => null, 'Eccube\Twig\Extension\IgnoreRoutingNotFoundExtension' => array('autoconfigure' => false), 'native_file_session_handler' => array('class' => 'Symfony\\Component\\HttpFoundation\\Session\\Storage\\Handler\\NativeFileSessionHandler', 'arguments' => array('%session.save_path%')), 'Eccube\Session\Storage\Handler\SameSiteNoneCompatSessionHandler' => array('arguments' => array('@native_file_session_handler')))), '/virtual/anju/public_html/motherearthresearchinstitute.org/shop/app/config/eccube/services.yaml')
in YamlFileLoader.php line 148
at YamlFileLoader->load('/virtual/anju/public_html/motherearthresearchinstitute.org/shop/app/config/eccube/services.yaml', null)
in FileLoader.php line 153
at FileLoader->doImport('/virtual/anju/public_html/motherearthresearchinstitute.org/shop/app/config/eccube/services.yaml', null, false, null)
in FileLoader.php line 90
at FileLoader->import('/virtual/anju/public_html/motherearthresearchinstitute.org/shop/app/config/eccube/services.yaml')
in GlobFileLoader.php line 27
at GlobFileLoader->load('/virtual/anju/public_html/motherearthresearchinstitute.org/shop/app/config/eccube/services.{php,xml,yaml,yml}', 'glob')
in DelegatingLoader.php line 40
at DelegatingLoader->load('/virtual/anju/public_html/motherearthresearchinstitute.org/shop/app/config/eccube/services.{php,xml,yaml,yml}', 'glob')
in Kernel.php line 140
at Kernel->configureContainer(object(ContainerBuilder), object(DelegatingLoader))
in MicroKernelTrait.php line 75
at Kernel->Symfony\Bundle\FrameworkBundle\Kernel\{closure}(object(ContainerBuilder))
in ClosureLoader.php line 38
at ClosureLoader->load(object(Closure), null)
in DelegatingLoader.php line 40
at DelegatingLoader->load(object(Closure))
in MicroKernelTrait.php line 78
at Kernel->registerContainerConfiguration(object(DelegatingLoader))
in Kernel.php line 776
at Kernel->buildContainer()
in Kernel.php line 642
at Kernel->initializeContainer()
in Kernel.php line 135
at Kernel->boot()
in Kernel.php line 110
at Kernel->boot()
in Kernel.php line 195
at Kernel->handle(object(Request))
in index.php line 76

以下のファイルを削除したら正常に動作するようになりました。何故、このようなファイルがそこに存在するようになったのは不明です。（FTPなどはしていないので）

src/Eccube/Request/atomlib.php

EC-CUBEのログは確認できますか？
どのようなエラーが出ますでしょうか？
var/log/prod/site-yyyy-mm-dd.log に「システムエラーが発生しました」といった行がありましたらご提示ください。

atomlib.php は WordPress のファイルのようですが、心当たりありますでしょうか？

https://github.com/WordPress/WordPress/blob/master/wp-includes/atomlib.php

もし、上記URLの内容と異なる内容でしたら、不正アクセスの懸念がありますので、くれぐれもご注意ください

お二方、コメントありがとうございます。
コメントに気づかずお返事が遅くなり申し訳ありません。

ご推察通り、不正アクセスによるものだったようです。
ある日突然、全ファイル削除されてしまい、頭もフォルダも真っ白になりました（大汗）