found the problem and fixed it... It is not what I thought at first.
I absolutely have no idea why this happened. It should have worked for everyone or for no-one, there is no logical reason why that was partially working

edit: one possibility: it failed only for those who did not visit the site just before the upgrade AND who had the auto-login option (hence why subzero had it fixed by cleaning cookies...)
But still I wonder why it was working on my local server and not on OVH's server.