In my case, rebooting the Mac and rebooting the Linux server does not offer assistance. This worked fine on El Capitan 10.11.4, but shows up broken in 10.11.5 Open Beta 1.
I can interface to the Linux server SMB offers from a Windows VM running on OS X, so I know the issue isn’t on the Linux side. Any thought what’s off-base, how to investigate the issue, or how to settle it?