permission denied when uploading a local file
 

the file permission is admin:sole-group = rw:rw

I add the user "support" into the group of "sole-group" and use it to upload the file, and got this error.

I suspect the group membership is not correctly configured. The id only shows the main group id of the user "support", though /etc/group shows the user "support" indeed is in the group of sole-group.

What could be wrong?

No idea, since you don't tell us how you're uploading the file, what the permissions are on the mount point, what the client is running, and what version/distro of Linux you're using on the server. Without details, we can't help.

I use ftp to upload the file "backup" interactively to an external ftp server. It's OK when I upload the same file under root.

Ok...so you're using FTP to upload the "backup" file, and you posted some things above, but don't say what they're for.

Are those the details for the CLIENT or the SERVER? Does the directory on the remote server have permissions for anyone but non-root people to write to it? What FTP server are you using? What client? Has this ever worked???

All info is from client side.

The file to be uploaded belongs to user/group admin/sole-group, so I added user support to group sole-group to have read access to it.
When I upload a file belonging to user support itself, it's no problem.
When I upload the same file under root, it's no problem.
For root and support, I use the same upload-user, so I think the problem is with the privilege of user support locally...

...which doesn't have anything to do with the permissions on the remote server.
The user id on the remote server has to have permissions on that server, to write to the directory on that server. Your local user/group doesn't enter into the equation at all.

Change the permissions on the remote directory to allow user writes. You still don't say anything about the remote server, or address any of the other questions asked, so there's little more we can say.

Let me clarify this more.

scenario one
============
login as root, start a FTP client connecting to remote FTP server. The credentials for FTP are upload/password for example. Upload the file. Works.

scenario two
============
login as support, start a FTP client connecting to remote FTP server. Use the same credentials as scenario #1, upload the file. Failed. error is
local: autobackup remote: autobackup
local: autobackup: Permission denied


So, the difference lies on the local, not remote side. Agree?

issue resolved. It turns out after adding user support to the same group of the file, I have to log into another session to make it take effect actually....