My files aren't syncing the way I would expect. How should I troubleshoot?

I have been using ownCloud for a week or so now. I installed my own server, and desktop clients on three machines. All running Ubuntu 14.04 and 16.04. So far this had worked admirably. However, apropos of nothing I can figure, one of my folders aren't syncing. Here is a copy from the desktop app log:

2016-09-16 10:24:53 AM, eclipse/workspace-halcon, ownCloud,Error downloading http://localhost:8080/owncloud/remote.php/webdav/eclipse/workspace-halcon - server replied: Locked

Steps to reproduce
1. create a new folder ~/ownCloud/eclipse/workspace-halcon

Expected behaviour
I would expect corresponding folders to be created on other sites.

Actual behaviour
Here is what appears in my log:
2016-09-16 10:24:53 AM, eclipse/workspace-halcon, ownCloud,Error downloading http://localhost:8080/owncloud/remote.php/webdav/eclipse/workspace-halcon - server replied: Locked

Client configuration
Client version: 2.2.3
Client operating system: Ubuntu 16.04

Server configuration
Operating system: Ubuntu 14.04
Web server: apache2
Database: sqlite
PHP version: php5
ownCloud version (see ownCloud admin page): 9.1.0
Updated from an older ownCloud or fresh install: fresh install
ownCloud log (data/owncloud.log):

The file is over 100MB long. Is there anything I should grep out of it?

Special configurations (external storage, external authentication, reverse proxy, server-side-encryption):
Did a basic vanilla install.

Hi,

see this FAQ containing all known info about this "server replied: Locked" issue: