We have 16.04 working here. We had to make changes to the following files
/etc/nsswitch.conf
/etc/ipa/default.conf
/etc/krb5.conf
/etc/ldap/ldap.conf
/etc/sssd/sssd.conf
/usr/share/pam-configs/mkhomedir
I'm guessing your nsswitch.conf is prob the issue if I had to guess. Here is what
ours looks like
#---------------------------------------
passwd: compat sss
group: compat sss
shadow: compat
hosts: files dns
networks: files
protocols: db files
services: db sss files
ethers: db files
rpc: db files
netgroup: sss
sudoers: sss files
#---------------------------------------