Hi. Sorry about that. The problem comes from the fact that we had to change cookie domain for security reasons - which means that for some users you need to delete old cookie otherwise it will keep interfering (your browser will keep sending it along with the new one).
Clearing cookies (all or just for opera.com domain) is the correct solution for now. But I will deploy a fix today that should solve this permanently for all - the downside being that it will log everyone out of the Forums.
Sorry again for all the inconvenience caused!
Permanent fix deployed. Please let me know if you still experience any problems! (The fix causes everyone to be logged out of the forums - that's expected and cannot be worked around).