I had this same problem. After contacting support, learned that I must be sure all Windows updates installed. Close TurboTax, update Windows (go to Settings>Updates). after updates completed and Windows restarted, open TurboTax and check for Updates (Online>Check for Updates). I was notified an actual update existed which I installed. TurboTax then restarted and I hit the Transmit Files button and it worked. All my tax forms transmitted. The person I spoke with at Support said if that didn't work I might have to uninstall and reinstall TurboTax. Apparently if you get stuck in the Update Loop when trying to transmit e-files, it's because TT can't find the actual update that needs to be installed, so updating Windows & then going directly to Update in TT allows the update mechanism to actually work.
