Together with the support team (thanks for the fast response time!) I was able to find out what the problem was and what the solution. The issue was that the user "myuser" had no access to /var/lib/resilio-sync/. So what I did: I added myuser to the group rslsync and set the permissions of /var/lib/resilio-sync/ to 775. That worked.
I am having Kubuntu 14.4. When I try to add a folder via a key, I get the Status "Database error". Reading the article "Unknown error (Database error)" did not help. I added the relevant log lines and the configs I use. I changed the beginning of resilio-sync to DAEMON=/usr/bin/rslsync SYNC_USER=myuser CONFIG=/etc/resilio-sync/config.json Can you help?