The issue is as follow :
It happens when we(me and some of my friends) try to login to our account, after filling the boxes and pressing the button, then it shows a page saying the following page is not found, eror 404(i added an image below). For example it always happens when i try to enter my codeforces account using steam's in-game browser, and sometimes when using opera browser, it also happens when trying to use my gmail to login. Also it always happens when my friend try to enter his account using his mobile phone.
I hope it helped codeforces staffs to improve the site.
Its fixed now, really appreciate it, now i can play and code in the same time :D.