The sequence is I would login to forum. select forum of interest click on the "Start New Disscusion" button and be redirected to the page that acknowleged that I had logged in."https://auth.opera.com/account/login/success?service=forums" instead of the new discussion composition editor.
That sequence was the same with firefox untill I added the cloudfront site to noscripts list. BUT even turning javescript permision on globaly wouldn't let me post to opera forums with opera???.
What about cookies? Do you have them enabled?
Yes. Always set to accept all cookies but to delete them when I close browser.
Hmm ... 12.17 (Windows) seems to be fine. Probably a caching issue. Opera 12.x had issues sometimes with caching of redirects, emptying the cache should fix it for a while.