Can't access admin - 503 service unavailable

<%- if @topic_view.topic.tags.present? %>
<%= t 'js.tagging.tags' %>: <%- @topic_view.topic.tags.each do |t| %> <%= t %> <%- end %>
<% end %>

Silverstripe Version: 4.8

Question: I’m updating my website from 3.x to 4.8, just keeping the database.
On my local machine (installed via composer) everything is ok.
On production (installed via softaculous) dev/build is ok but I’m not able to acces to admin.

I go to https://www.mysite.org/Security/login?BackURL=%2Fadmin%2Fpages
and I got this message :
503
Service Unavailable
The server is temporarily busy, try again later!
No login form.

But the website is ok, I can see all the pages.

Any idea ?

So my conclusion. SS4 installation via Softaculous (and even via FTP) doesn’t work.
I tried 3 times and impossible to access to the admin panel.

Installing SS4 via composer worked at the first time…