Fact is, that any version (including latest version 3.0.23 … which already announced that XFCE integration is supoorted) of Insync3 does not support thunar plugin on any Ubuntu-based Linux distro. The main reason is unavailability of thunarx-python package in proper version at Ubuntu repositories.
So, what is the current status of development of the insync3 Thunar plugin for XFCE desktop with any Ubuntu-based distro???
This is not joke guys, this is serious Insync flaw!!!
P.S. Your official client support is completely quiet regarding this problem for few weeks now.