DropboxServerException (nginx): 404 Not Found (File not found)

I'm developing an android app with Dropbox API (SDK 1.6.3).

I have a problem to get file from dropbox (There's no problem a few months ago.)

Folder structure (1 file and 1 folder)

Libro+ (root directory)

- libro.db

- images (directory that contains image files)

When I access libro.db to download, it raises 404 not found, but I can download this file via web browser and dropbox app.

Before downloading file, I got the metadata for the file.

I've tested it with two different dropbox accounts (A, B).

Strangely, this problem occurs in one account (A).

Is it an account problem ?

Thanks.