情報ありがとうございます。
デバッグモードでサイトにアクセスしましたところ、次のログが出力されました。

BadMethodCallException in EntityRepository.php line 226:
Undefined method 'setApp'. The method name must start with either findBy or findOneBy!

in EntityRepository.php line 226
at EntityRepository->__call('setApp', array(object(Application))) in GmoEpsilonServiceProvider.php line 133
at TaxRuleRepository->setApp(object(Application)) in GmoEpsilonServiceProvider.php line 133
at GmoEpsilonServiceProvider->Plugin\GmoEpsilon\ServiceProvider\{closure}(object(EntityManager), object(Application)) in Pimple.php line 201
at Pimple->{closure}(object(Application)) in Pimple.php line 126
at Application::{closure}(object(Application)) in Pimple.php line 83
at Pimple->offsetGet('orm.em') in GmoEpsilonServiceProvider.php line 100
at GmoEpsilonServiceProvider->Plugin\GmoEpsilon\ServiceProvider\{closure}(object(Application)) in Pimple.php line 126
at Application::{closure}(object(Application)) in Pimple.php line 83
at Pimple->offsetGet('eccube.plugin.epsilon.repository.regular_product') in GmoEpsilonServiceProvider.php line 143
at GmoEpsilonServiceProvider->register(object(Application)) in Application.php line 178
at Application->register(object(GmoEpsilonServiceProvider)) in Application.php line 906
at Application->loadPlugin() in Application.php line 610
at Application->initializePlugin() in index_dev.php line 59


> ここは通常の処理ですね。
そのようですね。
すみません、私のほうがソース内コメントを勘違いして読んでおりました。