O site não carrega com 2.4.5 e atribuição estável

No seu app.yml, você clona a branch estável do plugin?

Sim, assim:

hooks:
  after_code:
    - exec:
        cd: $home/plugins
        cmd:
          - git clone https://github.com/discourse/docker_manager.git
          - git clone https://github.com/discourse/discourse-spoiler-alert.git
          - git clone https://github.com/discourse/discourse-styleguide.git
          - git clone --branch stable https://github.com/discourse/discourse-solved.git
          - git clone https://github.com/discourse/discourse-user-notes.git
          - git clone https://github.com/discourse/discourse-checklist.git
          - git clone --branch stable https://github.com/discourse/discourse-assign.git