468様
ありがとうございます！ログすらどこにあるかわからずにいまして…。

見た所「epsilon.log」のログではこちらを繰り返していました。

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2018/08/22 06:47:21 [/shopping/load_payment_module.php] order_id:9 request settlement from 14.3.153.55
2018/08/22 06:47:21 [/shopping/load_payment_module.php] order_id:9 send request url:133.130.64.144 data:Array
(
[contract_code] => 65661890
[user_id] => 1
[user_name] => ○○○○○○○
[user_mail_add] => xx@xxx.xx
[order_number] => 9
[item_code] => roll
[item_name] => テーブルロール×3個 (代表)
[item_price] => 1680
[st_code] => 10000-0000-00000
[mission_code] => 1
[process_code] => 1
[xml] => 1
[memo1] =>
[memo2] => EC-CUBE_20180822064721
)
from 14.3.153.55
2018/08/22 06:47:21 [/shopping/load_payment_module.php] order_id:9 get response url:133.130.64.144 data:Array
(
[0] => Array
(
[tag] => HTML
[type] => open
[level] => 1
[attributes] => Array
(
[XMLNS] => http://www.w3.org/1999/xhtml
)

)

[1] => Array
(
[tag] => HEAD
[type] => open
[level] => 2
)

[2] => Array
(
[tag] => META
[type] => open
[level] => 3
[attributes] => Array
(
[HTTP-EQUIV] => Content-Type
[CONTENT] => text/html; charset=UTF-8
)

)

[3] => Array
(
[tag] => META
[type] => open
[level] => 4
[attributes] => Array
(
[HTTP-EQUIV] => Content-Style-Type
[CONTENT] => text/css
)

)

[4] => Array
(
[tag] => META
[type] => open
[level] => 5
[attributes] => Array
(
[HTTP-EQUIV] => Content-Script-Type
[CONTENT] => text/javascript
)

)

[5] => Array
(
[tag] => TITLE
[type] => complete
[level] => 6
[value] => 404 Error - Not Found
)

[6] => Array
(
[tag] => STYLE
[type] => complete
[level] => 6
[attributes] => Array
(
[TYPE] => text/css
)

)

)
from 14.3.153.55

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また、「error.log」では同時刻にこちらの記述があります。

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2018/08/22 06:47:21 [/shopping/load_payment_module.php] Warning(E_USER_WARNING): 従来互換用の HTTP_Request が読み込まれました。 on [/export/sd219/www/jp/r/e/gmoserver/2/2/sd1062522/files/eccube2/data/module/Request.php(30)] from 14.3.153.55
2018/08/22 06:47:21 [/shopping/load_payment_module.php] Warning(E_USER_WARNING): ページ情報を取得できませんでした。 on [/export/sd219/www/jp/r/e/gmoserver/2/2/sd1062522/files/eccube2/data/class/helper/SC_Helper_PageLayout.php(58)] from 14.3.153.55
2018/08/22 06:47:21 [/shopping/load_payment_module.php] Warning(E_USER_WARNING): メイン部のテンプレートが存在しません。[/export/sd219/www/jp/r/e/gmoserver/2/2/sd1062522/[hogehoge.jp]/../files/eccube2/data/Smarty/templates/bootstrap/.tpl] on [/export/sd219/www/jp/r/e/gmoserver/2/2/sd1062522/files/eccube2/data/class/helper/SC_Helper_PageLayout.php(65)] from 14.3.153.55

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お伝え忘れていましたが、bootstrapというテンプレートを使用しています。

何かわかりますでしょうか？
テンプレートが存在しないことだけはわかりましたが…