How to get TDS Sync Working again after Upgrading to HTTPS
When in doubt, start over
In our quest to upgrade this site to HTTPS, we ran into a fair number of challenges, the last of which (for now) was the fact that TDS was no longer able to sync in our dev environment. The error message that was produced was no help whatsoever. Unfortunately we didn't grab a screen cap of that error because we were able to solve the problem fairly quickly. The error message wasn't a real error message, meaning that it was unhandled or unexpected by Visual Studio / TDS. What I can tell you is that we got the red 'X' of death and a blurb of raw HTML spit out into the dialogue box.
At first we thought that the problem was the Sitecore Web Url (Located in Properties --> Build), which we did forget to update, but the problem still persisted. What we actually had to do was acquire a new Sitecore Access Guid and reinstall the Sitecore connector.
All the best! - Marcel