Seafile 9.0.15 web hang when upload above 400 folders via upload link

We have to upload many folder to Seafile Server Pro Seafile 9.0.15. To increase upload speed, we create upload link follow this instruction: Sharing files and folders - Seafile User Manual

And we drag and drop folders to browser after access upload link.

Folders and files upload ok but when to above 400 folders, Seafile 9.0.15 not upload run anymore. I don’t know how to fix this and how to see logs about this. How can I upload many folders via upload link?