Comment 11 for bug 1729337

FWIW I can confirm that adding sec=ntlmsspi fixed our issue with 4.4.0-98.

root@client:~# mount | grep share
//server/share on /mnt/share type cifs (rw,nosuid,nodev,relatime,vers=3.0,sec=ntlmsspi,cache=strict,username=www-data,domain=ourdomain,uid=33,forceuid,gid=33,forcegid,addr=10.4.0.30,file_mode=0770,dir_mode=0770,nounix,mapposix,rsize=1048576,wsize=1048576,echo_interval=60,actimeo=1,user)

root@client:~# uname -a
Linux client 4.4.0-98-generic #121-Ubuntu SMP Tue Oct 10 14:24:03 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux