"Your eReader is Full" message when both Dropbox and Calibre-Web are set up - no sync

[TS-CUBED]

Describe the bug/problem

Kobo Sage shows "Your eReader is Full" message when using Calibre-Web sync when Dropbox is enabled.

To Reproduce
Steps to reproduce the behavior:

Factory reset eReader
Setup Dropbox integration
Change api_endpoint to the Calibre-Web API-Endpoint (https://brandonjkessler.com/technology/2021/04/26/setup-kobo-sync-in-calibre-web.html)
Synchronise Kobo eReader

Expected behavior
eReader should sync the Shelf that has been setup for synchronisation.

Environment (please complete the following information):

OS: Pop!_OS Linux - Ubuntu 22.04 - kernel 6.2
Python version: 3.10
Calibre-Web version: 0.6.14
Docker container: None but happened in Docker container as well

Additional context

Problem does not occur if I do not set up the Dropbox, but change api_endpoint directly after initial setup. Interestingly the Kobo does not offer the Dropbox integration anymore after the api_endpoint has been changed.

With the Dropbox integration set up, changing the api_endpoint will result in this errror, and then, after a restart of the reader, the api_endpoint in the conf-file is reset to the default kobo one.

janeczku/calibre-web#2717