Cerbot - create SSL certificate : IOError: Unable to save to file

Question

For one of my website, I want to create SSL certificate with Let's Encrypt and Certbot on Debian 8 (Apache).

For it, I use the following command-line:

./certbot-auto --apache certonly -d example.com -vvv

Sadly, I get this error message:

  File "/opt/eff.org/certbot/venv/local/lib/python2.7/site-packages/certbot_apache/augeas_configurator.py", line 150, in save
    self.aug.save()
  File "/opt/eff.org/certbot/venv/local/lib/python2.7/site-packages/augeas.py", line 488, in save
    raise IOError("Unable to save to file!")
IOError: Unable to save to file!
An unexpected error occurred:
IOError: Unable to save to file!

Despite some research, I can't figure out a solution to solve it.