The problem occurs after once I tried to connect my university's Wifi which requires a login. The Opera opened and redirect to the login page, but failed to load it maybe because the poor connectivity and at the same time I create a new tab and go to a website (music.163.com) to test whether the network is connected or not. Obviously it did not make it. However, I cannot open the latter website ever since even connected to the good Wifi. because Opera seems to automatically redirect to the former website (the login page) according to the address bar. While the other browsers can correctly open music.163.com. I checked the host file and it does not seem to go wrong.