Nextcloud version 12.0.0
Operating system and version NAS4Free 11.0.0.4
Web server: lighttpd 1.4.45
PHP version: 7.1.5
Is this the first time you’ve seen this error?:
No
Can you reliably replicate it? (If so, please outline steps):
Yes
The issue you are facing:
I added NextCloud to a recent NAS4Free installation at home. Nextcloud runs, I created an admin user and password, I can get into it. I figured I shouldn’t have my default user as admin however, so I tried creating another user. It asked me for my password, and then repeatedly kept asking, never approving or denying.
I know the password is right, because I logged in with it. Also, if I type any other password, I actually get a message about failed authentication. This just repeats the same password dialog (although it eventually turns into a yes/no dialog instead of confirm/cancel, with the same message). (The log messages below are evidently from actual failed attempts and can’t be from the problem I’m having.)
I Googled and found several other instances of people who couldn’t create users, but all of those had simple solutions that don’t work for me. I then noticed that I can’t create groups or indeed do anything that requires authentication; in every case, I get the repeating password dialog.
At first I thought the problem might be related to the fact that I was using a data directory that wasn’t under www/nextcloud/. It seemed to work, but the external data directory had nothing in it, which I thought was weird because the one under www/nextcloud has other files, including a database. But I re-installed (after removing all the old directories) and used the default directory with the same results.
The output of your Nextcloud log in Admin > Logging:
Warning Login failed: ‘admin’ (Remote IP: ‘192.168.0.116’) 2017-06-22T19:13:22-0400
Warning Login failed: ‘dcroxton’ (Remote IP: ‘192.168.0.116’) 2017-06-22T19:04:22-0400
Warning Login failed: ‘dcroxton’ (Remote IP: ‘192.168.0.116’) 2017-06-22T19:04:19-0400
Warning Login failed: ‘dcroxton’ (Remote IP: ‘192.168.0.116’) 2017-06-22T19:04:17-0400
Warning Login failed: ‘admin’ (Remote IP: ‘192.168.0.116’) 2017-06-22T18:57:53-0400
Remember, this information may be requested if it isn’t supplied; for fastest response please provide as much as you can 
Feel free to use a pastebin service, otherwise log files can be indented with 4 spaces on each line to present them in a friendlier way on the forum.