I just unsuccessfully tried setting up a brand new Jenkins instance on a brand new server. Everything worked fine, except it's not possible to create user accounts, at all. The issue is the same when using the setup wizard ("create first new user") as when going to Manage Jenkins -> Manage Users -> Create new user.
The symptom is that no matter what is entered in the form (leaving all fields blank has the same effect as filling in everything correctly), when you press submit you simply get a HTTP 200 response. No error messages, nothing in jenkins.log, simply nothing happens.
I reverted to 2.121.3 using "apt-get install jenkins=2.121.3" and everything worked just fine.