Errors I see in the web console in Firefox:
======================================================
Loading failed for the <script> with source “http://localhost:8080/assets/webpack/runtime.a56338dd.bundle.js”. sign_in:33
Loading failed for the <script> with source “http://localhost:8080/assets/webpack/commons~pages.ldap.omniauth_callbacks~pages.omniauth_callbacks~pages.sessions~pages.sessions.new.2a29bfd9.chunk.js”. sign_in:35
Loading failed for the <script> with source “http://localhost:8080/assets/webpack/pages.sessions.new.32a7312a.chunk.js”. sign_in:36
Loading failed for the <script> with source “http://localhost:8080/assets/webpack/main.63ee71c8.chunk.js”. sign_in:34

I found the files it's looking for here:

me@mygitlab:/opt/gitlab/embedded/service/gitlab-rails/public$ ls -lah
total 92K
drwxr-xr-x  3 root root 4.0K Sep  7 02:04 .
drwxr-xr-x 24 root root 4.0K Sep  7 02:04 ..
-rw-r--r--  1 root root 3.1K Aug 28 12:52 404.html
-rw-r--r--  1 root root 2.9K Aug 28 12:52 422.html
-rw-r--r--  1 root root 2.9K Aug 28 12:52 500.html
-rw-r--r--  1 root root 2.9K Aug 28 12:52 502.html
-rw-r--r--  1 root root 2.9K Aug 28 12:52 503.html
-rw-r--r--  1 root root 9.3K Aug 28 12:52 apple-touch-icon.png
-rw-r--r--  1 root root 9.3K Aug 28 12:52 apple-touch-icon-precomposed.png
drwxr-xr-x 44 root root  20K Sep  7 02:03 assets
-rw-r--r--  1 root root 2.6K Aug 28 12:52 deploy.html
-rw-r--r--  1 root root 2.0K Aug 28 12:52 robots.txt
-rw-r--r--  1 root root 9.3K Aug 28 12:52 slash-command-logo.png
lrwxrwxrwx  1 root root   36 Sep  7 02:04 uploads -> /var/opt/gitlab/gitlab-rails/uploads
me@mygitlab:/opt/gitlab/embedded/service/gitlab-rails/public$