You don't tell us which version of Modx, Opera or OS you are using.
I just logged into a revo 2.2.1 install with Opera 11.62, in OS 10.7.3 and it works.
Oh Sorry, Yeah it's Modx revolution 2.2.1 and Opera 11.62. I am running windows 7. After the install the login page just refreshes non-stop.
I have the same environment on my PC and it works.
In my Opera no widgets and plugins. Try turn off widgets and plugins or reinstall Opera.
discuss.answer
Oh!! I figured it out. It was because Opera Turbo was enabled. Weird. Thanks everybody.