After I had my Nokia N9 early this month, I setup my Twitter account first — after some basic phone settings, of course. Everything worked. The application is probably not the best, but it works.
Is it problem-free app? I thought so, until I got a problem: I can’t connect to Twitter using this app. I was not sure when the problem occurred for the first time. I did played with my phone (installing apps, downloading musics, adjusting settings, and including working from the command line). When an update was available, I also upgraded.
Added later: I was not sure what caused this problem. When I signed-in for the first time, everything worked without any issues. I deleted my Twitter account from the app, and tried to add it again. I forgot when I did this for sure, before or after installing the firmware update. It  was optional, but I decided to upgrade.
At first, I was not sure about the problem. Is it my phone, or the internet connection I was using. I switched to some different connections, but it didn’t solve my problem. When I tapped the “Sign In” button, it said: “Can’t connect to Twitter”.
So, I searched for a solution. And, I got one. It’s probably something too technical, but the instruction is easy to understand. According to the forum thread, here are some steps to fix the problem — I’m using Software Version: PR1.1 (20.2011.40-2_PR_005):

• Activate “Developer mode” by going to: Settings > Security > Developer mode. It’s OFF by default. Turn it ON.
• After having it activated, you will find a “Terminal” app button under application menu.
• Open it and you need to execute these commands (one command per line):
  • gconftool --recursive-unset /system/http_proxy
  • gconftool --recursive-unset /system/proxy
  • gconftool --recursive-unset /system/osso

You may close your Terminal.
Those commands will wipe your active connection settings (bluetooth and internet data). But, it’s not a problem as I can add/manage connection settings easily. Problem fixed. Great.