CentOS - Feb 2017 - All Samba shares suddenly becoming read only

I have a Centos 7 server ? ZEPPO - which I use for file sharing. The clients 
are all Win7 PC's and I use Samba to serve the files. It has been working 
fine ever since I put it live late last year. Now, all of a sudden, all files 
are read-only.

I recently added a USB WiFi dongle to give the server a presence on the WiFi 
VLAN. Using this I am able to provide an Air Print service for our mobile 
devices.

For some reason this made the server unavailable to some clients, including a 
Fedora box (GROUCHO) that does host health checks using ping.

What I found was that if the WiFi was up on Zeppo (10.6.103.1/24) then Groucho 
(10.6.1.1/24) could not ping Zeppo's LAN address 10.6.1.101/24.  Groucho
only
has the LAN and has no WiFi.

However, Zeppo could still ping groucho.

If the WiFi was down on Zeppo then Groucho could ping Zeppo. Also, the clients 
could access the file shares.

Although I didn't get to the bottom of it, I did make it go away by

* removing an obsolete alias on the LAN interface 10.6.1.3/24
* changed the WiFi from DHCP to manual, although still with the same settings.

Once I'd done this, I was happy until the users started telling me that they
could only open documents as read-only. On testing this I found that all of 
the Samba shares are now read only.

I can't even create a new empty file in an existing folder.

I have not changed any file permissions on the server, nor have I changed and 
Samba config. 
I've had a look in /var/log/samba and can't see anything. 
	The log.smbd and log.nmbd don't show anyting relating to this.
	The log.10.6.5.2 file is empty
I've tried disabling selinux as a test but that made no difference.

The samba user = the linux user and if I ssh in as that user I can create 
files and folders.

Can anyone suggest what I need to do next to stop these shares being read 
only.

Ta
-- 
https://fundraise.cancerresearchuk.org/page/garys-march-march