I had a similar problem recently with Opensuse Leap 15.0 and Insync on one of my two machines. Got the same error “No protocol specified. insync.py: cannot connect to X server :0”.
With a little debuggin I found that the problem was that on my machine Xauthority files had the machine hostname without any domain, whereas uname reports the hostname with domain. The same difference seems to be in the strace provided in an earlier reply.
In my case I managed to solve the problem simply by adjusting my network settings.