@emranhot We've confirmed the problem and will investigate the issue. If you don't mind using non-Extreme data savings mode, it's possible to login basic authentication pages.
Hey listen this problem started only Opera Mini 21 for Android or Opera Mini 21 beta for Android. I love to use Extreme data savings mode. Please solve this problem as soon as possible. This is serious bug. We all want to view password protect web site in opera mini. Thanks, @miyukiwork