エラーログの確認ができました。
こういう機能があることを知らなかったので、ありがとうございます。
エラー内容は下記の通りでした。
Fatal error(E_USER_ERROR): Smarty error: [in C:/xampp/htdocs/ec-cube/eccube-2.13.3/html/../data/Smarty/templates/default/user_data/attach_mail.tpl line 145]: syntax error: unclosed tag \{else} (opened line 132). (Smarty_Compiler.class.php, line 317) on [C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\module\Smarty\libs\Smarty.class.php(1094)] from ::1
customer_id =
C:\xampp\htdocs\ec-cube\eccube-2.13.3\html\user_data\attach_mail.php(211): LC_Page_User->process
C:\xampp\htdocs\ec-cube\eccube-2.13.3\html\user_data\attach_mail.php(33): LC_Page->sendResponse
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\class\pages\LC_Page.php(155): SC_Display->prepare
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\class\SC_Display.php(80): SC_View->getResponse
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\class\SC_View.php(105): Smarty->fetch
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\module\Smarty\libs\Smarty.class.php(1264): include
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\Smarty\templates_c\default\%%F1^F15^F158E3F4%%site_frame.tpl.php(98): Smarty->_smarty_include
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\module\Smarty\libs\Smarty.class.php(1870): include
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\Smarty\templates_c\default\%%58^587^5876E692%%site_main.tpl.php(39): Smarty->_smarty_include
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\module\Smarty\libs\Smarty.class.php(1868): Smarty->_compile_resource
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\module\Smarty\libs\Smarty.class.php(1423): Smarty->_compile_source
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\module\Smarty\libs\Smarty.class.php(1490): Smarty_Compiler->_compile_file
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\module\Smarty\libs\Smarty_Compiler.class.php(317): Smarty_Compiler->_syntax_error
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\module\Smarty\libs\Smarty_Compiler.class.php(2256): Smarty->_trigger_fatal_error
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\module\Smarty\libs\Smarty.class.php(1816): Smarty->trigger_error
C:\xampp\htdocs\ec-cube\eccube-2.13.3\data\module\Smarty\libs\Smarty.class.php(1094): trigger_error