I was having the same issue and the resolution for me was proper white space in the gitlab.yml under the LDAP section. In fact, I'm using the omnibus installation so I made my configuration file changes in the /etc/gitlab/gitlab.rb file and then run gitlab-ctl reconfigure to generate the gitlab.yml.
I traced the cause of my problem though the unicorn std-err log which pointed to the file /opt/gitlab/embedded/service/gitlab-rails/config/initializers/1_settings.rb
The error message was:
block in <top (required)>': undefined method' for nil:NilClass (NoMethodError)
load'block in load'
The code under this section is looking at the ldap settings. In googled the error message and I found a reference to proper white space settings in the gitlab.yml file. I indented the section as follows (spaces, not TABS):
main: # 'main' is the GitLab 'provider ID' of this LDAP server
method: 'plain' # "tls" or "ssl" or "plain"
bind_dn: 'CN=svn bind,OU=Service Accounts,OU=RAL,OU=Users,OU=ID,DC=IDCorp,DC=com'
I reran gitlab-ctl reconfigure and it was fine.