We are currently aware of an issue which is causing some issues with the creation of new devices in the Sliding Sync Proxy. Some users may find that clients such as Element X may never finish logging in or loading the home screen. Logging out and back in again may be sufficient to work around the problem.
Clients that use the standard/old sync mechanism ('Sync v2', /v3/sync) and clients using the new native Simplified Sliding Sync support are not affected.