The port forwarding problem is huge with the online interface. Basically what you need to do is check two things:
1) Check your windows firewall - by default BB should add exceptions to your firewall but it may not always do this. In XP; go to Start -> Control Panel -> Windows Firewall -> Exceptions (tab at the top). Check to see if Blood Bowl or BB is listed here and if not you will need to manually "Add port" to configure the firewall to let BB 'get out' from behing it. There are 2 ports you need to open and you can find their details elsewhere (sorry I don't have them at hand)
2) Check your modem/router - this is more complicated and should only be done if step 1 doesn't improve things. This will require you to 'log in' to your modem/router and check the settings in it to see if it has any internal firewall also set up with the required ports open. As all modems and routers are different, try going here http://www.portforward.com
and finding your modem/router and folowing the instructions it gives you.
Now, as I am typing this form work I can't check this but I am sure it is right or at least pretty close.
If you browse to the BB install folder on your harddrive you should find a folder named Firewall (I think) or Firewall Assistant or something like that. Go into this folder and there will be an app named something like firewall config or firewall setup. Double click this and it 'should' automatically fix your windows firewall settings but probably not your modem/router.
Also make sure you have installed any and all available patches to your game.
And lastly, I too have expereinced the port forwarding error but it seems that this is the error code given also when the server is full. If you get into the lobby and check how many coaches are online and playing and the list is huge, from my expereince you can expect to get this error even if you have everything set up perfectly. Persistence in this case will usually pay off and if you manage to nab a slot as it becomes free your port issues miraculously disappear and you are somehow able to join games.
Hope this helps or that someone can give you more detail than I can from work