Confirm that all users (from whichever Desktop-GUI instances) are disconnected from the Abacus; if still encountering error, force off user-accounts that may be stuck in the buffer.
Please keep in mind that other users/Desktops that are running the same version of GUI-software as what is currently on your Abacus unit can connect to it, to use any available call-generation cards. This may contribute to your inability to update firmware.
To force a user-account that is stuck in the system (if everyone confirmed that they are not connnected, etc)...
1. Open the same version of GUI on your desktop, that is currently on the Abacus unit.
2. Go to View/System Information... and 'release all' the cards if you should have any cards currently acquired (as seen in the grid on the right-side panel of window.
2a. If you see any cards that seems to be acquired or is shown as 'RED', that means someone somewhere has it acquired still. You can lookup the IP address of that user/Desktop via the 'SC Users' tab (lower-left sub-window).
3. Before further pursuing whoever maybe connected to this Abacus unit, try and DELETE the user (under this SC Users' sub-window). Select the user/IP address and click on 'Delete user'.
4. At this point, if listed user(s) cannot be deleted, you will need to figure out who that user/IP address is and convince them that they ARE still connected; have them try and disconnect again.