Okay, I just tested this on our forum and it works properly.
The above points to our "General" category node and the main tab works as well as all child nodes when using the tab.
I am actually wondering if it has something to do with the way you have your URLs setup (or with how your server handles URL anchors)... it looks like the URL you gave as an example which resulted in the 404 "oops" page is trying to visit an HTML anchor instead of the node itself.
Can you right click the "IT Service Desk" nav tab and copy/paste the URL for it (you can strip out the IP, just leave everything else in it - but, do the right click copy/paste method, not by clicking the nav tab and copy/pasting the resulting URL from your browser URL bar)...
...and can you also visit the actual "General IT Support" category... actually, scratch that. Let's try something.
Can you go to your admin panel, then Options -> Node & forum list... then the bottom option where it says "Create pages for categories" - check this box and save.
Then refresh your forum main page try clicking the navigation tab for IT Service Desk again. Does that result in it working properly?
That will help me debug what I was talking about above regarding HTML anchors.