Thanks for you reply.
Actually my version was 2.7.3.1375, and I thought I would be noticed about any updates.
Installed 2.7.3.1381, but the problem remains.
I had to stop and start the app, then one test file was synced, but when I removed that file it was not removed from all peers like expected.
The app on the NAS works fine for every file I add to all other peers. I'm using the NAS for saving files from my picture scanner.
Is there any log file that could help?