Cannot read files on Linux client

Hallo,

I have a weird problem with a Linux client trying to access a Samba share
available on a Linux server (an Asus WL500gp router running OpenWrt Kamikaze
8.09).  Though I can navigate through the shared directory tree, all files
have "rwxrwSrwx" permissions, and I get a "setgid executable, regular file,
no read permission" error when I try to read them.

Note that I have no problem accessing and reading files of this share from
an Xbox client running XBMC.  The problem happens only on a Linux client.
This is the output of "uname -a":  (I am running Kubuntu Jaunty)

"Linux localhost 2.6.30-020630rc2-generic #020630rc2 SMP Wed Apr 15 13:20:18 UTC 2009 x86_64 GNU/Linux"

And this is the smb.conf:

[global]
syslog = 0
syslog only = yes
workgroup = WORKGROUP
server string = WL500GP
preferred master = yes
domain master = yes
security = share
encrypt passwords = yes
guest account = nobody
local master = yes
name resolve order = lmhosts hosts bcast

[myshare]
path = /mnt/sda1/myshare
browseable = yes
writeable = yes
read only = no
valid users = user1 user2 user3
read list = user3
write list = user1 user2
force user = user2

Any ideas on what the problem could be?

Thank you very much!
Jean