[Samba] intermittend problem since samba 4 upgrade

[murrxx.1 at arcor.de]

Since the upgrade to samba 4 our Ubuntu suffers from broken connections.
Sometimes users can connect to the samba shares for a while but mostly it fails.
Problem is that our windows AD domain is a main user authentication domain.
It is centralized and we have no influence on that. 
Let's call it domA.
Our Linux systems are in a one way trusted resource domain "domB" and we use users.map to map the linux users to the windows domain users.
DNS wise domA has a central domain extension like domA.central.com
domB on the other hand is domB.local.com
We had no problem with that up to the samba 4 upgrade.
Since then we frequently see errors like this:
tail -f log.wb-domA
  gss_init_sec_context failed with [ Miscellaneous failure (see text): Server (cifs/hostA at DOMB.LOCAL.COM) unknown]
  [2016/11/08 11:16:40.758724,  0] ../source3/librpc/crypto/gse.c:341(gse_get_client_auth_token)
  gss_init_sec_context failed with [ Miscellaneous failure (see text): Server (krbtgt/DOMA.CENTRAL.COM at DOMB.LOCAL.COM) unknown]

Our smb.conf holds:
   workgroup = DOMB
   realm     = DOMB.local.com
   security = DOMAIN
   password server = *

Any clues how to troubleshoot the problem?