After one day of extensive research on the internet, I still can't figure out this one. What is happening is that PHPSESSID in the cookie isn't persisting from one page to another.
This is happening in computers under one internet connection in a specific place. I've tested several others machines located elsewhere and everything works fine. I've checked session_start, there's no problem with subdomains.
I hope someone can help me out.
分享 After I verified with my own computer that this was happening in this place, I realised that when I typed some URLs, they got somehow changed in the middle, for example http://www.medicmanager.com.ar/cloud/santiago/charisma/js/jquery.cookie.js was changed into http://www.medicmanager.com.ar/cloud/santiago/charisma/js/jquery.ckiooe.js, which of course gave 404.
I changed the ISP connection to another, and it started working, so I called the ISP and they told me they've been with this problem for 2 days, changing URLs and messing up with cookies.
Thank you all for your answers, and I leave this here in case someone is in doubt. Yes, it can be your ISP, very strange.
分享
