Ok, I figured it out in the chat with the help of @markvanlan, @sam, @joffreyjaffeux, @REALITY:
Summary
I had the topic list previews plugin still installed on my site and despite it not being enabled, it was causing a problem on the server side.
I removed it from the app.yml file and did a rebuild and it works again.
And then added it back as a theme component per this:
Thank you for the help!
Details
(found by appending /logs to my instance’s base URL in the browser)
StandardError (Attempted to access the non preloaded custom field 'topic_list_tiles'. This is disallowed to prevent N+1 queries.) app/models/concerns/has_custom_fields.rb:173:in ’ (eval):35:in _fa
app/models/concerns/has_custom_fields.rb:173:in `[]'
plugins/discourse-topic-previews/plugin.rb:78:in `block (3 levels) in activate!'
(eval):35:in `_fast_attributes'
active_model_serializers (0.8.4) lib/active_model/serializer.rb:456:in `attributes'
plugins/discourse-custom-wizard/extensions/custom_field/serializer.rb:4:in `attributes'
active_model_serializers (0.8.4) lib/active_model/serializer.rb:480:in `_serializable_hash'
active_model_serializers (0.8.4) lib/active_model/serializer.rb:359:in `serializable_hash'
active_model_serializers (0.8.4) lib/active_model/serializer.rb:347:in `as_json'
plugins/discourse-chat/app/serializers/chat_channel_serializer.rb:38:in `chatable'
(eval):4:in `_fast_attributes'