@mardiyah, of course, I tried with all the extentions off, no change
@leocg, I've got Chrome, it acts just the same: Opera and Chrome say ERR_CONNECTION_CLOSED.
Meanwhile Firefox shows a hint saying (not literally) 'You have to go to a spectial page to use this network' and a button 'Open the page'. Then it goes to their detectportal.firefox.com/canonical.html, which somehow redirects to our local address, which today was:
http://c3.glhs.ru/pr/2c83d107b74e5a0ec77b48b06ba95d1e/index?pi=3166 (it will be different the other time). There I press 'Go to Internet' of our university, and all the programs use the Internet.