WOW. Finally solved this problem !!
I was using an auto-register page for guests and when I was trying to click on the Twitter login page, nothing was happening. It was just like it was refreshing the portal.
So, I tried to create a Web Login page instead, and when I tested i got an error :
"Desktop applications only support the oauth_callback value 'oob'"
Figured out that it was because I left the Callback URL field blank on my Twitter App...
This HAS to be non-empty
I put "https://example.com" only to have something in there and everything worked perfectly.
After several weeks of scratching my head, I'm happy I found what was going on and hope it will help others that are trying to implement Twitter for their guest login.
- nice2k