I am having this error whenever I try to log in on Launchpad and Ask Ubuntu with browsers as Opera, Comodo Dragon (my default on Windows) and Chrome. Only Firefox seems to work. Why is this happening and how to fix it? (I have cookies allowed.)
Do you use Launchpad Single Sign-On Service to access Ask Ubuntu? That's the most obvious technical connection between the sites, especially since you say this problem is happening when you try to log on.
If so, then I recommend posting a question about this on the Canonical SSO provider support tracker on Launchpad. (You should search first, and be aware that this might be judged as a bug; you might be asked to create a bug report, or your question on Launchpad might be converted into one.) This is the recommended way to get support for Ubuntu SSO and Launchpad SSO.