Intermittent problem accessing samba shares

[Paul Marwick via arch-general <arch-general@xxxxxxxxxxxxx>]

Fully updated Arch install, running Xfce. I don't use Samba very much, 
but need it for access at various times, so I have gvfs, gvfs-smb and 
smbclient installed. Normally no problem accessing Samba shares from 
Thunar. However....

Recently I spent some time trying to work out why a client was unable to 
access Samba shares from an Xubuntu desktop machine (shares are on an 
Ubuntu 16.04 server). While I was testing, I did something (wish I knew 
what...) and ended up with Thunar suddenly being unable to access any 
Samba share, even though I could use mount.cifs to mount the same share.

When I got back to my home network, I found similar problems, even 
though I'd never had any problems in that area with this install of 
Arch. At the same time, another Arch install on the same laptop had no 
problems accessing the shares, and my desktop machine was also still 
able to access the shares without any problems.

The problem is strange and very variable. Sometimes it fails as soon as 
I click "browse network" - the system sits and then comes back with 
'unable to access "/" timeout was reached'. Sometimes I get a display of 
the Windows network icon, then it fails to open the Windows network 
display. Other times, I am able to see and mount a share on the network. 
This is on the same network, with no changes made to the network (one 
Slackware-based server, one Ubuntu 16.04 server). And a network which 
other Arch installs have no problems accessing the shares.

I've checked messages using journalctl  and dmesg and see nothing when 
the access fails. The part I find really puzzling is that sometimes it 
works with any problems, other times it fails as soon as I start.

I'm confident that the prolbem is not Thunar itself. I installed pcmanfm 
to see if a different file manager would behave differently, but it 
shows exactly the same problem - sometimes it works, other times it fails.

I'm at a loss as to how I track this error. I'd really like to fix it, 
so I'm hoping someone can suggest what might cause the problem, and how 
to fix it?

I've seen problems like this in the past (mainly on Debian based 
distros, though a few years ago on an Arch install). One of a very small 
number of problems I've never been able to predictably fix.

Paul.