I have seen something similar to this before when using nginx to reverse proxy to a sub-folder of a domain rather than the root of a domain or sub-domain.
If your APP_BASE_URL contains the sub-folder "joplin" (e.g. APP_BASE_URL=https://example.com/joplin) and your nginx server block defines a redirect location of /joplin, when nginx passes the request to the server it appends /joplin and so the server sees the request as https://example.com/joplin/joplin, hence the path not found message.
I helped someone in the past by devising a rewrite in the nginx server block to stop the duplication. In their case the sub-folder was named "joplinServer" not "joplin".
See this post:
I am in no way an expert with regards to this. If anyone knows a better way please post here!!!