Failed to add /run/systemd/ask-password to directory watch: No space left on device?

Does anyone knows why have I this message with the new update of samba on ubuntu 16.04.1?

Paramétrage de samba (2:4.3.9+dfsg-0ubuntu0.16.04.3) ...
Failed to add /run/systemd/ask-password to directory watch: No space left on device: 

Since I have so much space, I don't understand :

df -h
Sys. de fichiers                  Taille Utilisé Dispo Uti% Monté sur
udev                                 16G       0   16G   0% /dev
tmpfs                               3,2G     11M  3,2G   1% /run
/dev/sda2                           107G     49G   53G  48% /
tmpfs                                16G    184K   16G   1% /dev/shm
tmpfs                               5,0M    4,0K  5,0M   1% /run/lock
tmpfs                                16G       0   16G   0% /sys/fs/cgroup
/dev/sdi2                           367G    343G  5,2G  99% /media/divers
/dev/sda1                           110G    366M  104G   1% /opt
/dev/sdm1                           147G    136G   11G  93% /media/nfsmedia/syno/usb4
/dev/sdq1                            74G     69G  1,1G  99% /media/nfsmedia/syno/usb8
/dev/sdp1                           459G    453G  5,6G  99% /media/nfsmedia/syno/usb1
/dev/sde2                           735G    684G   14G  99% /media/series
/dev/sdo1                           1,8T   1015G  726G  59% /media/nfsmedia/syno/usb3
/dev/sdr1                            74G     68G  1,6G  98% /media/nfsmedia/syno/usb7
/dev/mapper/RAIDSTOCK-RAID5FSTOCK   9,0T    7,3T  1,4T  85% /media/RAIDFORSTOCK
/dev/mapper/RAID1FORDOCK-DOCK       550G    303G  220G  58% /media/DOCK
cgmfs                               100K       0  100K   0% /run/cgmanager/fs
tmpfs                               3,2G       0  3,2G   0% /run/user/1004
//192.168.6.12/vigilian             1,9T    1,7T  179G  91% /media/smbseries/nsa
//192.168.6.11/NASA                 930G    807G  123G  87% /media/smbseries/nasa
tmpfs                               3,2G     12K  3,2G   1% /run/user/123
tmpfs                               3,2G       0  3,2G   0% /run/user/1000

As discussed in a Red Hat bug report, it turns out that the Crashplan back-up service is the most likely culprit. It uses many inotify watches and, eventually, eats them all up.

The immediate fix is to run:

sudo -i
echo 1048576 > /proc/sys/fs/inotify/max_user_watches
exit

to make more watches available.

The long-term fix is to edit the file /etc/sysctl.conf to include the line:

fs.inotify.max_user_watches=1048576

I'm not reputable enough to comment on the accepted answer, but I wanted to say that it's by no means limited to CrashPlan. Dropbox and other file sharing platforms use inotify watches per inode to detect when an upstream sync needs to occur. Malware detectors may have watches on directories. Other backup tools besides CrashPlan might, as well.

To see what's consuming inotify watches, use lsof:

sudo lsof -K | grep inotify | (less||more||pg)