I just bought Insync and installed it on openSUSE Leap 15 but the GUI is not working. The tray icon appears briefly and disappears. I downgraded to version 1.5.2 and now it works.
Judging by the multiple posts on this issue, version 1.5.5 seems to affect a lot of people on different distros. You should seriously consider removing this version from the download page while you fix the issue.