Temps de démarrage très long sur le serveur Ubuntu (configuration réseau)

J'ai essayé de résoudre deux problèmes que je soupçonne d'être liés. Récemment, notre réseau a été reconfiguré et, par la suite, j'ai rencontré un temps de démarrage très long en raison de problèmes de configuration réseau et, après le démarrage, je ne pouvais plus accéder au partage de fichiers samba à partir d'un client Win10.

Les messages de l'heure de démarrage :

[  OK  ] Reached target Network (Pre).
Starting Network Service...
Starting Wait for Network to be Configured...
Starting Network Name Resolution.
[  OK  ] Started Network Name Resolution.
[  OK  ] Reached target Host and Network Name Lookups.
[**    ] A start job is running for Wait for Network to be Configured (XX / no limit)

Il faut environ deux minutes pour que le dernier message soit effacé. Je pense que c'est peut-être un problème lié au DHCP ou au DNS. J'ai également remarqué plusieurs erreurs de type "NXDOMAIN / violation potentielle de DVE2018-0001" entrecoupées dans /var/log/syslog. Pour l'instant, je ne sais vraiment pas où aller chercher la cause de ce retard (ou pourquoi le serveur de fichiers n'est pas visible pour les clients Win10). Incidemment, Samba semble fonctionner et Webmin signale que le partage de dossiers est actif...

EDIT 1 :

Selon la suggestion de @heynnema dans les commentaires, la sortie de cat /etc/netplan/*.yaml :

# This is the network config written by 'subiquity'
network:
    ethernets:
        enp4s0:
            dhcp4: true
    version: 2

EDIT 2 :

Sortie relative à Samba.

Contenu de fstab :

# /etc/fstab: static file system information.
#
# Use 'blkid' to print the universally unique identifier for a
# device; this may be used with UUID= as a more robust way to name devices
# that works even if disks are added and removed. See fstab(5).
#
# <file system> <mount point>   <type>  <options>       <dump>  <pass>
# / was on /dev/md2 during curtin installation
/dev/disk/by-id/md-uuid-4c695059:2f774b0b:9e5492d2:bcb62609 / ext4 defaults 0 0
/dev/disk/by-id/md-uuid-05f3f447:6002213f:2bb97c81:6416f121 none swap sw 0 0
# /boot was on /dev/md0 during curtin installation
/dev/disk/by-id/md-uuid-58dd9ab9:74b02d8a:10890b55:2212c9cb /boot ext4 defaults 0 
0
/swap.img   none    swap    sw  0   0

smb.conf Définitions de partage :

#======================= Share Definitions =======================

# Un-comment the following (and tweak the other settings below to suit)
# to enable the default home directory shares. This will share each
# user's home directory as \\server\username
;[homes]
;   comment = Home Directories
;   browseable = no

# By default, the home directories are exported read-only. Change the
# next parameter to 'no' if you want to be able to write to them.
;   read only = yes

# File creation mask is set to 0700 for security reasons. If you want to
# create files with group=rw permissions, set next parameter to 0775.
;   create mask = 0700

# Directory creation mask is set to 0700 for security reasons. If you want to
# create dirs. with group=rw permissions, set next parameter to 0775.
;   directory mask = 0700

# By default, \\server\username shares can be connected to by anyone
# with access to the samba server.
# Un-comment the following parameter to make sure that only "username"
# can connect to \\server\username
# This might need tweaking when using external authentication schemes
;   valid users = %S

# Un-comment the following and create the netlogon directory for Domain 
Logons
# (you need to configure Samba to act as a domain controller too.)
;[netlogon]
;   comment = Network Logon Service
;   path = /home/samba/netlogon
;   guest ok = yes
;   read only = yes

# Un-comment the following and create the profiles directory to store
# users profiles (see the "logon path" option above)
# (you need to configure Samba to act as a domain controller too.)
# The path below should be writable by all users so that their
# profile directory may be created the first time they log on
;[profiles]
;   comment = Users profiles
;   path = /home/samba/profiles
;   guest ok = no
;   browseable = no
;   create mask = 0600
;   directory mask = 0700

[printers]
   comment = All Printers
   browseable = no
   path = /var/spool/samba
   printable = yes
   guest ok = no
   read only = yes
   create mask = 0700

# Windows clients look for this share name as a source of downloadable
# printer drivers
[print$]
   comment = Printer Drivers
   path = /var/lib/samba/printers
   browseable = yes
   read only = yes
   guest ok = no
# Uncomment to allow remote administration of Windows print drivers.
# You may need to replace 'lpadmin' with the name of the group your
# admin users are members of.
# Please note that you also need to set appropriate Unix permissions
# to the drivers directory for these users to have write rights in it
;   write list = root, @lpadmin

[media_Public]
    create mode = 777
    path = /home/media/Public
    writeable = yes
    public = yes
    directory mode = 777