DatabaseConnectionNotDefinedException: The specified database connection is not defined: default

have you enabled PDO ?

There is an exception catch here, but maybe the server environment is bypassing this somehow?

    try {
      if (function_exists('db_query')) {
        $file = db_query("SELECT filename FROM {system} WHERE name = :name AND type = :type", array(':name' => $name, ':type' => $type))->fetchField();
        if (file_exists(DRUPAL_ROOT . '/' . $file)) {
          $files[$type][$name] = $file;
        }
      }
    }
    catch (Exception $e) {
      // The database table may not exist because Drupal is not yet installed,
      // or the database might be down. We have a fallback for this case so we
      // hide the error completely.
    }

So this may be related #914742: Add hook_requirements() for buggy versions of eAccelerator. If disabling opcode caching allows installation, then this should be marked as a duplicate.

More likely #947616: Warn people not to enable xdebug.show_exception_trace will help.

[edit] This looks like a xdebug trace [/edit]

If it's #947616: Warn people not to enable xdebug.show_exception_trace I think we should re-open that and get the patch in.

Needs feedback from Xano now.