Just clean-installed Ubuntu 18.04 beta2 and Insync. Although Insync shows a green tick, it’s not actually doing anything.
I installed Insync 1.3.24 instead. And when Insync is running, I checked the settings and found that none of my folders are selected for sync’ing. That’s why Insync was doing nothing. I selected the folders, re-installed Insync 1.4.5. Now it’s working fine.