Description of problem:
The last security update for samba server introduced a new parameter 'client ipc signing' with a default to mandatory (compare https://www.samba.org/samba/security/CVE-2016-2115.html)
While the default for 'server signing = false' it leads to the situation that that a client which tries a RPC connection to a PDC will fail because the server doesn't support signing!

Using rpcclient to connect to a local server will fail like this:
cli_negprot: SMB signing is mandatory and the server doesn't support it.
failed negprot: NT_STATUS_ACCESS_DENIED
Cannot connect to server.  Error was NT_STATUS_ACCESS_DENIED

Setting 'server singing = auto' will solve that problem for rpcclient but may introduce other problems with e.g. windows clients:
When connecting with a Win7 client using user nobody (default for guest access) to a samba share the client will fail with the following error:
[2016/05/14 23:30:27.403950,  1] smbd/service.c:1114(make_connection_snum)
  vm-buero (192.168.39.10) connect to service austausch initially as user nobody (uid=65534, gid=600) (pid 27161)
[2016/05/14 23:30:27.404507,  1] smbd/process.c:457(receive_smb_talloc)
  receive_smb_raw_talloc failed for client 192.168.39.10 read error = NT_STATUS_CONNECTION_RESET.
[2016/05/14 23:30:27.404946,  1] smbd/service.c:1378(close_cnum)
  vm-buero (192.168.39.10) closed connection to service austausch


The only work around I found so far is to set 'client ipc signing = auto' which 
reduces the security for SMB signing from mandatory to offered but as the server signing is still disabled by default no signing is used....

The first part (regarding the regression) is also discussed upstream and seems to affect onyl samba 3.6 and older:
"It is only a problem with 3.6 and older, where we didn't implenent
the FLAGS2_SMB_SECURITY_SIGNATURES_REQUIRED logic."
Source: http://comments.gmane.org/gmane.network.samba.internals/90769

Unfortunately I found no reason / explanation why a Win7 client cannot connect using user = nobody with 'server singing = auto'. IMHO it should be possible because to manpage 'auto' means "SMB1 signing is offered, but not enforced." 

At least I'm not the only one with that problem, I found another report in debian forum unfortunately in German (https://debianforum.de/forum/viewtopic.php?f=9&t=160557). While that is for samba 4.2 it is due to the samba update fixing CVE-2016-2115