No space left on device: AH00023: Couldn’t create the ssl-stapling mutex

I was woken up yesterday morning by my service that checks the health of the server. I am an early bird but not that early, the good side was that nobody noticed the server went down at that hour so I didn’t have any pressure on my shoulders.

So I checked: all websites were down, cpanel/whm down, at least SSH was working well… so lucky… I tried to start the web server and checked the error log:
"No space left on device: AH00023: Couldn't create the ssl-stapling mutex"

No space?… so I checked the space, plenty of it…
After some digging I ended up solving the issue by:
vi /etc/sysctl.conf
# added this line:
kernel.sem = 250 256000 32 1024
# and then:
sysctl -p