Why is my Samba share only writable by root?

Question

I have set up a Samba share, and it is only writable by root. For example, I can only create a folder on that share with sudo:

~$ mkdir /mnt/SHARE/somedir
mkdir: cannot create directory /mnt/SHARE/somedir: permission denied
~$ sudo mkdir /mnt/SHARE/somedir
sudo: enter password
~$ ls /mnt/SHARE/somedir | grep some
somedir

What to do ? The weirdest thing is, it worked until yesterday without sudo, ad I didn't change anything !