There have been (rare? unique?) case when a plugin would cause an error even while disabled. Did you try to rebuild without any plugin in app.yml
?
Spending hours trying to pinpoint an issue is often frustrating, and re-creating the server, or trying to restore a backup on a temporary server to see if it works is sometimes faster, even if we don’t know where the issue originates from.
edit:
----------------------------DISCOURSE CHATBOT ERROR----------------------------------
Discourse Chatbot now requires the embedding extension on the PostgreSQL database.
See required changes to `app.yml` described at:
https://github.com/merefield/discourse-chatbot/pull/33
Alternatively, you can remove Discourse Chatbot to rebuild.
----------------------------DISCOURSE CHATBOT ERROR----------------------------------
Did you try this?