>1. I'm unable to chose a forum different than my default forum (the categories selection don't change).

This is probably related to a security in your browser about javascript functionality. If you work with showing javascript errors, do you see something in the page about it? Basically, when changing the forum dropdown list, this will initiate a request to the server to refresh the category. If the security is too high, this might not work.

>2. I can not create a thread to a member.

Could you elaborate more on this? This is highly generic and it doesn't give me any information as to where to this is happening and what is not working.

>3. Every time I click on a number of new messages the new extra tab opens.

...browser related, we talked about that before, I do not know what to recommend at this point for this.

>I'm using IE8. Is there anything I can adjust to fix these problems?

As IE8 been released in full production? I do not have it right now as I thought it was still not released. This might be caused by some new functionalities in it and I would have to look into that.