At least one valid file should be uploaded.

I had this error in Drupal 8.9.20. The error was occurring in two of the three website environments, although it was occurring in any web browser and user role.
The final solution for me has been #15. Specifically, I modified the dropzonejs.settings.yml configuration by adjusting:

tmp_upload_scheme: public

I face this issue with version 8.x-2.8, but on anonymous role, authenticated users have no problems.

My site has a form for guest user to submit a node with images. I use DropzoneJS for uploading images.

It works properly for logged in users. But for guests, it always complains "At least one valid file should be uploaded".

Can somebody help?

Hi @tuthanh. I suggest you check the anonymous role permissions for the DropzoneJs module. If they are correct, check solution #15 💬 At least one valid file should be uploaded. Fixed .