Again Problems with WebDAV Sync

I use an on premise Synology appliance as file server. On this nas I created a share for webdav access and integrated it in my DTPO. The synchronisation looks fine there. No error messages and I configured to sync every 15 minutes. The status also gets update with the new time stamp when a new synchronisation was fulfilled.
Only when I’m in office I start my DT on the smart phone to intiate a synchonisation. The synchronisation is working well, but, and this problem, not all files will be download to the smart phone. Also groups are missing. I cannot find any logic behind that why some files and groups are not synchronized.

Do you have any suggestion how to find and solve the problem?

Thanks in advanced.

Is anything reported in Window > Log?

No messages in the Log.
I also tried the following:

  1. optimized database
  2. Sync a second mobile device --> same problem

Any further hints?

Hold the Option key and choose Help > Report bug to start a support ticket.