Thanks for your additional input. I’ve just done another attempt earlier today of using the plugin commit 3a6512d0560211b93f022a27ed7276024d0020dc with my by now upgraded discourse 3.3.3, but I ended up in a state where all HTTP requesets were answered with Completed 500 Internal Server Error in 42ms (ActiveRecord: 0.0ms | Allocations: 16860) no matter for which URL. There was nothing in the production.log, nothing erroneous logged in the standard output/error and nothing in the passenger.3000.log either. Removing the plugin from the plugin directory + restarting has resolved the problem. It was reproducible several times.
For now I’m giving up. It was a nice idea to play with the ActivityPub integration, but I guess it’s not at the level where I can get it to do anything useful, unlike other plugins I managed to install succesfully. After all, it’s not a critical requirement, just a “nice to have”.