I have InSync running on a Debian machine in a network that has only IPv6 available. IPv4-only hosts are available via a NAT64 service. Other applications successfully connect to both IPv4 and IPv6 hosts. Google’s web services, including Drive, work in both Firefox and Chrome.
InSync starts but does not synchronise any files. There are no messages in the “Errors” or “Actions required” sections. The only indication of a fault is that there is no green tick on the system tray icon.
Reintroducing IPv4 fixes the problem instantly.
My setup is a little unusual (at least in IPv4-rich parts of the world) so I would consider providing a system to test on if required.