samba - Nov 2019 - Subject=Re: Mac Clients Disconnect with: source3/smbd/service.c:1050(close_cnum) closed connection to service data

And sorry, to answer your question here's what the smb.conf's looked
like:

SERVERA - Ubuntu 18.04 (Samba 4.7.6) sudden issue:

Hi Louis,
> When you get a chance, it seems like a number of our excel files were
> corrupted. We can't seem to open them at all. I tried opening them from
R
> and SPSS.
> They give different errors:
> My error (mac):
> Excel cannot open the file 'Drug_exposure.xlsx' because the file
format or
> file extension is not valid. Verify that the file has not been corrupted
> and that the file extension matches the format of the file.
> Nick's error (PC):
> Access denied. Contact your administrator.

Problem: group read permissions were being lost, owner retained them. This
is with sticky bit already set.

[global]
workgroup = DEPT
security = ads
realm = DEPT.EXAMPLE.COM
netbios name = SERVERA
idmap config * : backend = tdb
idmap config * : range = 3000-7999
idmap config DEPT:backend = rid
idmap config DEPT:range = 10000-199999
winbind enum users = yes
winbind enum groups = yes
winbind use default domain = yes
winbind nested groups = yes
winbind refresh tickets = yes
winbind offline logon = true
winbind expand groups = 1
template homedir = /home/%U
template shell = /bin/bash
client use spnego = yes
client ntlmv2 auth = yes
log file = /var/log/samba/samba.log
log level = 2
;obey pam restrictions = yes
inherit permissions = yes
client ipc signing = auto
;wins support = yes
ea support = yes
vfs objects = catia fruit streams_xattr
fruit:metadata = stream
delete veto files = true
fruit:model = MacSamba
fruit:veto_appledouble = yes
fruit:posix_rename = yes
fruit:zero_file_id = yes
fruit:resource = xattr

[homes]
available = yes
comment = Home directories
read only = No
browseable = No

[data]
available = yes
comment = Main data store for all users
path = /data
valid users = +DEPT\groupa
force group = +DEPT\groupa
writable = yes
read only = no
create mask = 2770
force create mode = 2770
directory mask = 2770
force directory mode = 2770
access based share enum = yes
durable handles = yes
inherit acls = yes

SERVERB UBUNTU 16.04 (Samba 4.3.11):  Same, with these differences:

[data]
create mask = 0770
force create mode = 0770
directory mask = 0770
force directory mode = 0770

But no issues other than the icon in Finder turning into a question mark.

SERVERC - Ubuntu 18.04 (Samba 4.7.6) - No issues reported

[global]
workgroup = DEPT
security = ads
realm = DEPT.EXAMPLE.COM
idmap config * : backend = tdb
idmap config * : range = 3000-7999
idmap config DEPT:backend = rid
idmap config DEPT:range = 10000-199999
winbind enum users = yes
winbind enum groups = yes
winbind use default domain = yes
winbind nested groups = yes
winbind refresh tickets = yes
winbind offline logon = true
winbind expand groups = 1
template homedir = /home/%U
template shell = /bin/bash
client use spnego = yes
client ntlmv2 auth = yes
log file = /var/log/samba/samba.log
log level = 2
;obey pam restrictions = yes
inherit permissions = yes
client ipc signing = auto
;wins support = yes
ea support = yes
vfs objects = catia fruit streams_xattr
fruit:metadata = stream
delete veto files = true
fruit:model = MacSamba
fruit:veto_appledouble = yes
fruit:posix_rename = yes
fruit:zero_file_id = yes
fruit:resource = xattr

[homes]
available = yes
comment = Home directories
read only = No
browseable = No

[data]
available = yes
comment = Main data store for all users
path = /data
valid users = +DEPT\groupc
force group = +DEPT\groupc
writable = yes
read only = no
create mask = 0770
force create mode = 0770
directory mask = 0770
force directory mode = 0770
access based share enum = yes
durable handles = yes

All the best,
Louis (OP)