smbfs and access denied

Hi everyone, here's my problem
I'm using a xen/debian server with smbfs share 
"//serveu/folder     /mount/point/ smbfs
credentials=/root/.smbcredentials,fmask=660,gid=group        0       0"

Users are accessing to thoses share with their kubuntu (konqueror)
without any problems, they can do what they want under their /home/$.
BUT, when they copy/move files from their workspace to the share point
folder (/mount/point/), i've got a nice error :
"impossible to modify right access of /mount/point/a_file.txt)" (sorry
if there's a bad translation of the msg :) )

Thought, files are moved and rights changed. I've tried with other
browser such as rox-filer but it remains the same kind of error.
Although, there is no error msg with nautilus, or in bash cmd.

If someone knows about this error, i think it's a problem due to a bad
file managing with smbfs in konqueror.

thx in advance.