This is really a worst case experience for a new Nextcloud user. For a more detailed analysis, we’d need some more information, e.g. the versions of Nextcloud and the client you are using (the template that is shown when creating a new topic is a good starting point).
To generate logs of the deleting procedure:
You can start the client with logging more information into a file: https://docs.nextcloud.com/desktop/2.3/troubleshooting.html#log-files
On the server side, there are as well a few logs:
/path/to/nextcloud/data/nextcloud.log
/var/log/apache2/access.log
/var/log/apache2/error.log
A first test could be as well to take the client completely out of the scenario and do uploads via web-interface and webdav. You can as well test the client only with one of the test instances (don’t use any sensitive data):