oh the version of aim on that phone is for sprint i believe you have AIM 2.0 with 4.5 i believe? go download the older version of AIM i forgot the site its something like www.blackberry.com/aim or something or www.mobile.blackberry.com/aim
If you don't have docs to go installed I recommend installing it as it does cause issues if you don't. Not sure if it causes this issue or not but it does cause issues.
aim 2.0.43 doesnt work for the 8320 cause rim only has it enabled for the 8130 and 8330 devices....no matter what, you cant get it to work because RIM has to enable it first. and once they do that it will work...probaly when 4.5 os officially gets released...whenever that is