There is a known issue with Kaspersky causing a problem. Do you use Kaspersky ? If yes: If you're using Kaspersky, you don't have to touch anything for TB, but make some adjustments in Kaspersky:

Suggestions from Kaspersky support to confirm if the problem caused by Kaspersky : - Uninstall software potentially incompatible with Kaspersky ( eg:WinPcap and Glary Utilities).

- Open Kaspersky settings - Additional - Network and scroll at the bottom of the page. In Mozilla section, temporary uncheck the option: Scan secure traffic in Mozilla applications

Andy, you confirmed your issue was caused by Kaspersky?