You are not giving much information, so its hard to tell what could be causing the problem. The IGN ac utility forum is very helpful for errors like this. The most common problem people are having is if they are running Vista or Windows7 and they install AC into its default directory.
General rule with Vista or Win7. Do not install AC, Decal, or any decal plugins under the program files directory. Create a directory like c:\games and put everything under that.
c:\games
C:\games\asheron's call
C:\games\decal
c:\games\decal plugins
Just giving examples.
If it is a port issue then you can look at
http://www.portforward.com . This website lets your pick the router you have and then which game you want it to work with. It then walks you through step by step setting up forwarding on that router. (You should do what I stated up top first though because in most cases that fixes it.)
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