Amazon S3 Plugin Not Working

Hi, I am testing this plugin before use. Below is my test procedure:

  1. Create a fresh install of KB latest version
  2. Create a task and upload a file (no problem until here)
  3. Install AWS S3 plugin
  4. Create second task and upload a file

On step 4, KB shows error message below when I click “Upload Files” button
Unable to upload files, check the permissions of your data folder.

Obviously, there is nothing wrong with the permissions because my test is ok for step 2.
I also tested my AWS key-secret with another tool. AWS creds are also ok.

My settings for the plugin is below. I leave two fields empty, because it seems that they are optional on the plugin documentation.
awsSettings

I enabled debug, and below is the debug output. Again there seems no problem there.

Is there anyone using AWS plugin?
@Fred can you give me a clue to proceed?

[2020-10-28 22:06:43] [debug] ###############################################
[2020-10-28 22:06:48] [debug] Kanboard\Core\Controller\Runner::executeMiddleware
[2020-10-28 22:06:48] [debug] Subscriber executed: Kanboard\Subscriber\BootstrapSubscriber::execute
[2020-10-28 22:06:48] [debug] Kanboard\Core\Controller\BaseMiddleware::next => Kanboard\Middleware\AuthenticationMiddleware
[2020-10-28 22:06:48] [debug] Kanboard\Core\Controller\BaseMiddleware::next => Kanboard\Middleware\PostAuthenticationMiddleware
[2020-10-28 22:06:48] [debug] Kanboard\Core\Controller\BaseMiddleware::next => Kanboard\Middleware\ApplicationAuthorizationMiddleware
[2020-10-28 22:06:48] [debug] Kanboard\Core\Controller\BaseMiddleware::next => Kanboard\Middleware\ProjectAuthorizationMiddleware
[2020-10-28 22:06:48] [debug] Kanboard\Core\Controller\Runner::executeController => \Kanboard\Controller\BoardAjaxController::check
[2020-10-28 22:06:48] [debug] SQL: SELECT option, value FROM settings
[2020-10-28 22:06:48] [debug] SQL: query_duration=0.00053715705871582
[2020-10-28 22:06:48] [debug] SQL: total_execution_time=0.00053715705871582
[2020-10-28 22:06:48] [debug] SQL: SELECT data FROM sessions WHERE id = ? LIMIT 1
[2020-10-28 22:06:48] [debug] SQL: query_duration=0.0004730224609375
[2020-10-28 22:06:48] [debug] SQL: total_execution_time=0.0010101795196533
[2020-10-28 22:06:48] [debug] SQL: SELECT projects.id, projects.name FROM projects LEFT JOIN project_has_users ON project_has_users.project_id=projects.id WHERE project_has_users.user_id = ? AND projects.is_active IN (?)
[2020-10-28 22:06:48] [debug] SQL: query_duration=0.00043702125549316
[2020-10-28 22:06:48] [debug] SQL: total_execution_time=0.0014472007751465
[2020-10-28 22:06:48] [debug] SQL: SELECT projects.id, projects.name FROM projects LEFT JOIN project_has_groups ON project_has_groups.project_id=projects.id LEFT JOIN group_has_users ON group_has_users.group_id=project_has_groups.group_id WHERE group_has_users.user_id = ? AND projects.is_active IN (?)
[2020-10-28 22:06:48] [debug] SQL: query_duration=0.00029706954956055
[2020-10-28 22:06:48] [debug] SQL: total_execution_time=0.001744270324707
[2020-10-28 22:06:48] [debug] SQL: SELECT * FROM actions WHERE project_id IN (?)
[2020-10-28 22:06:48] [debug] SQL: query_duration=0.00015807151794434
[2020-10-28 22:06:48] [debug] SQL: total_execution_time=0.0019023418426514
[2020-10-28 22:06:48] [debug] SQL: SELECT * FROM action_has_params
[2020-10-28 22:06:48] [debug] SQL: query_duration=0.00011897087097168
[2020-10-28 22:06:48] [debug] SQL: total_execution_time=0.002021312713623
[2020-10-28 22:06:48] [debug] SQL: SELECT 1 FROM users WHERE id = ? AND is_active = ? AND role = ?
[2020-10-28 22:06:48] [debug] SQL: query_duration=0.00019288063049316
[2020-10-28 22:06:48] [debug] SQL: total_execution_time=0.0022141933441162
[2020-10-28 22:06:48] [debug] SQL: SELECT COUNT(*) FROM projects WHERE id = ? AND last_modified > ?
[2020-10-28 22:06:48] [debug] SQL: query_duration=0.00020384788513184
[2020-10-28 22:06:48] [debug] SQL: total_execution_time=0.002418041229248
[2020-10-28 22:06:48] [debug] SQL: SELECT 1 FROM sessions WHERE id = ?
[2020-10-28 22:06:48] [debug] SQL: query_duration=0.00013995170593262
[2020-10-28 22:06:48] [debug] SQL: total_execution_time=0.0025579929351807
[2020-10-28 22:06:48] [debug] SQL: UPDATE sessions SET expire_at=?, data=? WHERE id = ?
[2020-10-28 22:06:48] [debug] SQL: query_duration=0.00055885314941406
[2020-10-28 22:06:48] [debug] SQL: total_execution_time=0.0031168460845947
[2020-10-28 22:06:48] [debug] APP: nb_queries=10
[2020-10-28 22:06:48] [debug] APP: rendering_time=0.066221952438354
[2020-10-28 22:06:48] [debug] APP: memory_usage=5.33M
[2020-10-28 22:06:48] [debug] APP: uri=/aws/?controller=BoardAjaxController&action=check&project_id=1&timestamp=1603922617&_=1603922618272
[2020-10-28 22:06:48] [debug] ###############################################