In fact, I didn't see how I can change the users for NFS. I mean, I can mount the folder, but it doesn't ask me for login/pass .. and all my users are in the "fileshare" group.
Moreover, the /var/www has rw for everyone.
All the users (on the server and on the client) are in apache and filesharing groups, and it doesn't change anything.
Finally, everything in the NFS options (via webmin) seem correct (from what I understand ..).
PS: and it doesn't explain why one of the clients works like a charm, and not the other o_O