It looks like this is working now, so will answer my own question for this thread. Whether this is the best answer, no idea, but it solved the problem I had.
If your app shares DB Inputs with the DB Connect App, make sure that user perms are added to your app, so that that the built-in DB Connect users have the necessary rights in your own app. I added these users to my app, whether they need read/write will depend on your app:
db_connect_admin
db_connect_user
In the end, I turned on SSL to avoid any scenarios whereby functions are dependent on REST API calls. If stuck, it's worth doing this to see if that helps. It did for me.
N.
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