Thank you for your replies Andrius and Ewald. I installed the same versions of VirtualBox, Debian, and Syncthing on my iMac and everything works perfectly! However, on my MacBook running VirtualBox/Debian I get a “Connection Error” message on the Syncthing GUI every time I try to add a device. I am using the bridged network adapter on VirtualBox. The problem must be a hardware/software bug between VirtualBox and the MacBook. I will search for the multicast/broadcast packet drop problem. Any more troubleshooting suggestions?