I made browsing work again by creating /etc/samba/smb.conf and adding those lines:

client ipc min protocol = NT1
client ipc max protocol = SMB3
client min protocol = NT1
client max protocol = SMB3

Now I still can't start the samba daemon with the default config...

Have you tried giving the Help Center Samba documentation a quick look?

Also, what is your use case? Why are you attempting to start samba.service (which is the Active Directory daemon)?

My mistake : I confused the samba service and the smb service :(

Still, should the error "Samba detected misconfigured 'server role'" for the samba service be checked?

sudo testparm doesn't throw an error for the server role = standalone server configuration parameter?

$ sudo testparm
Load smb config files from /usr/share/defaults/samba/smb.conf
rlimit_max: increasing rlimit_max (10000) to minimum Windows limit (16384)
Processing section "[homes]"
Loaded services file OK.
Server role: ROLE_STANDALONE

Press enter to see a dump of your service definitions

But sure, we can land it in unstable for the next sync. It won't change the fact that smbd will still be configured to run as Server role: ROLE_STANDALONE, though (I checked).

In fact, the samba (AD) service complains exactly because the config is standalone. For samba.service to even work, the server role needs to be set to e.g. server role = active directory domain controller per sudo systemctl status samba.service output.