error 403 when trying to log in during checkout It's the first time i'm posting on a forum but i've been really stuck with this problem for a few days.
So thanks for your help.

I'm working on a site that's entirely https.
The site uses the anonym checkout  possibilities. I recently observed that when a user wants to connect during checkout (using the "/user" link, I don't use extra pannel to log in), after adding a product to cart or after a purchase, he gets an error 403.
Whatever role he has, he is stuck with this error. The login and the password are correct. And this doesn't happen when we are on another page. It only appears from the moment a user adds a product in the cart.
Then he has to empty the session and cache of the navigator to connect to his account. 

I don't understand the problem because I don't have this problem on localhost, the only difference i see between the two server is that i don't use https on the localhost.
That's why i think it's a session problem but i'm not sure.
I tried all the solutions proposed for the settings.php, I don't use secure page (i used to but i uninstalled it because the site is entirely https now).
The site is on a subdomain.

I have made a redirection to htacess to redirect possible entry by http to https with this code:
RewriteCond %{HTTPS} off
RewriteRule ^(.*)$https://a.domain.com/$1[R=301,L]  

I don't know what else to do, thanks for your help. Sorry if there's a lack of information, just tell me.

Thanks.
My problem was a session bug with my version of php (5.6.3) the code from here contains a workaround: 

function mymodule_user_login(&amp;$edit, $account) {
  $_SESSION['mymodule']['ulogged'] = 1;
}