Aha! Thanks to your help I have now managed to fix this issue! I did in fact have an error in the logs, but at server start up (when it attempts to make that POST request you mention).
The error message was “The .forestadmin-schema.json file does not exist” - despite it being present in the deployed code bundle.
To fix this I referred to another post in the forum here that advises the following:
In your Dockerfile, after “ENV NODE_PATH=...” add:
ENV FOREST_SCHEMA_DIR=/usr/src/app
And then in middlewares/forestadmin.js after “authSecret: process.env...”:
schemaDir: process.env.FOREST_SCHEMA_DIR,
And my staging environment is up and running - many thanks!