アップグレードを試みましたが、何か間違えたようです

[JammyDodger]

There have been a couple of similar reports recently about the previews plugin being an old version, and updating that to its new repo fixes things:

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

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