Thanks Helen for your reply. The btsync application on FreeBSD is behind a firewall and within a jail. Despite I redirect incoming traffic on both ports (sync, webui) and allow all outgoing tcp/udp traffic the reconnect won't work. There seems to be something missing for the pf firewall settings (redirect, or something else). I also sent an email, so I got a ticket #41051. As soon as I got my passwort I will provide the logs and a more detailed description on the FreeBSD setup.
Thank you so much.
John