permissions - Not able to create user on Ubuntu 14.04 (32 bit)

I'm sorry this post is so long, but I wanted to make sure that all the stuff that might be relevant was readily available.

I've been trying to create a new user account on my laptop, and, even though I tried everything I found online, I'm still not able to do it. I would like to remark that I was not using uppercase letters (which seems to be the reason why some other users could not create a new account on the questions I checked).

First of all, I tried to create an account in GUI mode. After entering the new username on the account creation dialog I get this message:

running '/usr/sbin/adduser' failed: Child process exited with code 1

After some googling I downloaded gnome-system-tools from the Ubuntu Software Centre. I enter all the data on the "Create New User" dialog and this is what I get:

The configuration could not be saved

Invalid data was found.

That's when I decide to open the terminal and see what was going on. I enter the command sudo adduser newuser I get this:

adduser: The group `newuser' already exists.

so I delete the group using the command sudo delgroup newuser, and use the previous command again. After doing so, I get this:

Adding new group 'newuser' (1001) ...
Adding new user 'newuser' (1001) with group 'newuser' ...
useradd: existing lock file /etc/subgid.lock without a PID
useradd: cannot lock /etc/subgid; try again later.
adduser: '/usr/sbin/useradd -d /home/newuser -g newuser -s /bin/bash -u 1001 newuser' returned error code 18. Exiting.

Again, the newuser group is created, but not the user. However, when using the useradd command instead of adduser, I just get the two useradd lines above and no group is created. In neither case I'm able to create a new user. I also tried all these commands opening the terminal as root and it was useless. What should I do now?

Edit: I rebooted the machine twice, and repeated the process using the terminal as root. It did not work.