i am having the same problem. Server 2012R2 as DC01, 2016 as DC02.
The main issue i have with the AD join page is that there's no "test credentials" function. i have no idea if it's a real error or human error. Nor are there any (that i can find with any ease) documentation as to what is expected in the fillable fields under the SMB "Active Directory Settings" (noted below).
other Linux servers have joined before and way back when i was on my first unraid (running 5.x OS) tower 1.x (i am now on T3.0 [running 6.7.2])
as a last test i created a copy of my domain admin account called it t3admin, then created a AD security group called t3users. this was to eliminate the possibility of a code bug where a space in any of the names was causing an issue.
::note for admins to review the code:: Solved, but not really solved.
ok, in the middle of writing this my friend and i were able to join our T3 to the AD domain. it might be a fluke, but something to note is that after, or maybe in the process, of submitting the join attempt the SAMBA service/process either doesn't re-load or crashed upon re-load. this would cause share browsing issues and also AD join. note the below format of the fields i used and in the middle of joining my friend started SAMBA when it when down
User: t3admin
AD security group: t3users
::note it's not best practice, but both of the above user and group are in the root "Users" OU. i note this for replicating the test criteria if needed::
DNS FQDN: t3.mydomain.com
AD domain name (FQDN): mydomain.com
AD short domain name: myworkgroup
AD account login: t3admin
AD account password:
AD initial owner: t3admin
AD initial group: t3users
Any questions feel free to ask. i will try to help and answer.
Pete T.