Oops. The software powering this discussion forum encountered an unexpected problem

[schungx (Stephen Chung)]

Could be this thing!

I have this in my app.yml

git clone https://github.com/merefield/discourse-topic-previews.git -b "theme_sidecar"

This is obviously an old repo with a special branch!

Will try with the latest one:

git clone https://github.com/paviliondev/discourse-topic-previews-sidecar

EDIT: It works!!!

Thanks a great deal to @raghukamath