sudo useradd: cannot lock /etc/passwd; try again later (no lock files)

When doing this on Ubuntu server 14.04:

camilstaps@localhost:~$ sudo useradd --home-dir /var/www/xxx.camilstaps.nl/ --no-create-home --root /var/www/xxx.camilstaps.nl/ --shell /bin/sh xxx

I'm getting the error:

useradd: cannot lock /etc/passwd; try again later.

I checked and tried the following (found the suggestions all over the internet):

• Obviously, I'm using sudo
• There exists a file /etc/.pwd.lock, but it is empty and removing it doesn't help
• There are no other *.lock files in the /etc directory
• The filesystem is not full (14G available on /)
• Rebooting the server doesn't remove the lock.

I'm stumped. How can I unlock /etc/passwd?

df -ih output

camilstaps@localhost:~$ df -ih
Filesystem     Inodes IUsed IFree IUse% Mounted on
/dev/vda1        1,3M  228K  1,1M   18% /
none             125K     4  125K    1% /sys/fs/cgroup
udev             124K   404  123K    1% /dev
tmpfs            125K   386  124K    1% /run
none             125K     5  125K    1% /run/lock
none             125K     1  125K    1% /run/shm
none             125K     2  125K    1% /run/user

You can use adduser instead of useradd. adduser also provides an interactive session to configure your Linux user and also creates the necessary /home/username/ directory automatically.

sudo adduser <username>

By the way, there is no difference between adduser and useradd. They both serve the same purpose.