In my experience this is usually an RSLinx Classic issue. Is this a new error on a system that was working or has it always done this?
I would shutdown RSLogix, and then shutdown RSLinx. RSLinx can be sneaky when you try to close it. IF you just close the window it is sometimes still running in the system tray (down by the computer clock). Look in the system tray and see if you see the little chain-link looking icon. If you do, right-click on it and select the option to Shut RSLinx down. Not sure of the exact wording.
If you get a message that other programs are using it then that tells us something else is using RSLinx. Maybe FT View, but it doesn't need it typically.
Anyway, make sure RSLinx shuts down completely and then re-open RSLogix. When you click on the menu option "Comms" ==> "System Comms" RSLinx will re-open. See if you get the error again.
OG