Me too! I decided to finally upgrade to Insync 3 and immediately regretted it. Many of these same issues (minus the “Error”).
It wouldn’t even log me in at first. Then eventually the “Add a Google Account” button worked and I got the Chrome window that said " You should now be logged in. You may now close this page. If you haven’t bought a license already, you can check out our pricing plans [here.]". HOWEVER, it would just stay on “Add a Google Account” page and not go anywhere. After many attempts it finally let me go through the rest of the linking steps but never synced anything.
So then I decided to try uninstalling that and installing 1.5 instead…now same issue. After MANY tries on at least two different computers and networks it is the same thing. It will sometimes eventually link but I haven’t had anything sync at all in either version on any computer - IF I can even get it to link to my account to begin with.
I absolutely loved Insync for the longest time and now since a few months ago - things were not syncing right and now not at all.