Logged-in but creating new thread impossible

Actually, it's always a cookie issue whenever the next page or click on the forum results in you being logged out when previously you were logged in. But, cookie management can involve a lot more than just whether your browser is set to allow cookies or not. Any other cookie management, proxy software, software firewalls, brower plug-ins or add-on modules, and so on, that can effect the control and management of cookies can be at fault.

For example, it's not enough to allow session cookies but not permanent ones because forum software like this uses a brief 15 minute session timeout. Some add-ons and browser/privacy software tweaks convert permanent cookies to session cookies, but that causes you to be logged out on this type of forum if you haven't clicked any link for more then 15 minutes. The session cookies expire and your login credentials are lost.

The new thread you mentioned above is very detailed. If it took you a second longer than 15 minutes to compose it, and for some reason your browsing is making use of session cookies, you will be logged out when you hit the submit button. Following that post, you made two more, (a new thread - this one, and a reply to the wish-list thread), and do not appear to have timed out for those posts, so that's why I'm assuming you've got something blocking or converting permanent cookies.