I believe that all of the issues you've just referred to, our developer team is aware of and currently working on.
They've found a workaround for IE users to be able to sign in while they find a permanent fix, and that will be noted on the sign-in page with instructions for IE users.
The 404 errors on conversation pages have been fixed now. Developers are working on fixing the search issue.
The few conversations with blank titles are also something they're working to fix.
The frequent logouts people have been experiencing are related to the work the developers are doing -- every time they push a change to the live site, it resets the sessions and folks will have to log in again. It's not a permanent thing.
In the meantime, we have fixed the All Topics list, which now works as expected, with subtopics dropping down below when you click the topics. The list of topic areas is now visible right from the Community home page, just as it was on the old forum, so people should find it easier to get around now. If you'd like, you can also customise which modules appear on your homepage by going to My Account -> Edit Profile and ticking or unticking the relevant boxes.
Thank you very much for your patience as we fix these bugs and also make improvements.