a problem in the attachments :( ?!

[LeQatar]

Salut..
I have a problem with the attachments in my forum.
Some members can't attach files.
When they upload any file, they see like this picture:

note that:
1- I have given all of my members unlimited size of uploading.
2- Some members can upload and others can't!?
3- all files ( attachements ) are stored in the server not in the database.
4- I have given 777 cmd for all folders of the attachments.

is there any way to fix it ?

Thanx alot
LeQatar