Same on KDE Neon (Ubuntu 16.04)
OS:
$ lsb_release -a
No LSB modules are available.
Distributor ID: neon
Description: KDE neon User Edition 5.9
Release: 16.04
Codename: xenial
Insync:
$ insync -v
1.3.15.36150
Output (https://pastebin.com/n8UQh6aU):
$ insync start --no-daemon
Downgrading to 1.3.14 solves the issue.
Download links: New Insync version: 1.3.14