With an unstable internet connection, the default behavior of the JavaScript DNS library resolver results in at some point marking all DNS servers as unreachable, even if it was only a temporary problem.
To get the DNS resolution to work again, one has to either manually reset the status (by changing the DNS settings or restarting TB), or change a hidden setting.

The default value should be reconsidered, and the setting probably be exposed in the GUI.

Related: #90