I have saved one NC 12 server with all user accounts data in a system that the power supply stop working, and the replacement power supply was obsolete, even on ebay, to make it even harder it was an IDE HDD.
You maybe able to start a Live Ubuntu’s file manager in sudo mode and copy all your data to a new HDD as a backup for a fresh install.
That’s what I would do.
You need to change the permissions of all folders and files because you have just copied them as root in the GUI. The GUI may have a problem working in a Live image environment or with another system from my experience.
I have basically the same thing to do on my current NC 25 server that is at 96% disk usage and is shutdown atm.
How I rescued an old NC 12 server.
I ended up booting up my rescue IDE HDD PC with a Ubuntu Live DVD with the old IDE HDD attached.
Do the update.
Do the upgrade.
I then installed just SSH but not the server. It gets there as an automatic part of the SSH install.
You edit the ssh config to allow root and restart the service.
I then transferred the files via SSH to another PC.
Note: Don’t leave a browser open because it causes the Live image to crash. It did it to me halfway into the backup.
I setup a New install of NC 24 server on another PC and then synced all the user accounts data to the New NC 24 server.
This whole process took about 2 weeks to complete.