Thanks for the quick response @pmusaraj. Before posting I successfully added a couple of themes. Just added Topic Stopwatch in case something is broken/different re theme components. That installed cleanly with no issues. Also added the Kanban addon that was in the automatic “popular” list with no issues.
I also tried installing the DiscoTOC refactor branch last night and had the same issue.
I Used https://github.com/discourse/DiscoTOC as the URL and manually set the branch name from master → main.
All it gives me is a server error 500 
Trying the two year old branch marks-testing installs fine (!)
Dashboard shows Discourse version is 2.8.0.beta11 (cbf99f48d8).
Plugins installed and active: discourse-chat-integration, discourse-voting, discourse-formatting-toolbar, discourse-graphviz, discourse-hot-topics, discourse-math, discourse-solved, discourse-user-notes, discourse-whos-online, docker-manager. Also styleguide plugin is installed but not active.
Let me know if there’s any other info I can collect to help debug this!