There were issues with our Signal proxies due to a change Signal made to shut down some of their backend services which the proxies connect to. These old hosts were working for some time, but recently stopped working. We've updated our proxies with the latest configurations, and the proxies should be working once again.
Change that Signal made: https://github.com/signalapp/Signal-TLS-Proxy/commit/732f0f09fe2103f9f47c609bc84f7edb85d6b5c8