API session expiry?

These sessions don't expire, nor do the access tokens themselves. (Note that access tokens can be explicitly revoked though, e.g.,via https://www.dropbox.com/account/security .) Further, the App Console lets you generate an access token for your account and app, but it will only show you that token when you generate it. If you return to or reload that page, the previously generated token won't still be showing. Generating another won't revoke the first one though.

Anyway it sounds like at a certain point, some Dropbox API calls stopped returning the result you expected, or they started failing. So, to investigate this issue, can you share the error/output you got from the app when it started failing? The corresponding code may also be helpful.