Comment 15 for bug 1729337

That did it for me on Xenial.

root@client:~# uname -a
Linux client 4.4.0-98-generic #121~lp1729337 SMP Thu Nov 2 20:53:20 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux

root@client:~# mount | grep share
//server/share on /mnt/share type cifs (rw,nosuid,nodev,relatime,vers=3.0,sec=ntlmssp,cache=strict,username=www-data,domain=ourdomain,uid=33,forceuid,gid=33,forcegid,addr=1.2.3.4,file_mode=0770,dir_mode=0770,nounix,mapposix,rsize=1048576,wsize=1048576,echo_interval=60,actimeo=1,user)

Interestingly, as FedeX pointed out, the option (unspecified in my fstab) of sec=ntlmssp has appeared as a new explicit parameter. Makes no difference to me, and it works, just FYI.