Current Nightly on Windows and Linux. Getting a "Cannot connect to the server for 14 days" error. https://services.mozilla.com/status/ claims all is fine. Reported by many other people in #developers. Additionally, "Tabs from other devices" in the history menu stays grey. If I force a sync in the menu, it finishes instantly, doesn't change the "last sync di 22:35" message, and "tabs from other devices" never un-grays.

Related bugs: Bug 1048193 and Bug 1048376

1407265481911 Sync.ErrorHandler DEBUG Flushing file log. 1407265481913 Sync.Service DEBUG Exception: No keyFetchToken No traceback available 1407265481913 Sync.Service DEBUG Not syncing: login returned false. 1407265481913 Sync.BrowserIDManager INFO currentAuthState returning error.login.reason.network due to previous failure 1407265481913 Sync.Status DEBUG Status.login: error.login.reason.network => error.login.reason.network 1407265481913 Sync.Status DEBUG Status.service: error.login.failed => error.login.failed 1407265481921 Sync.BrowserIDManager INFO currentAuthState returning error.login.reason.network due to previous failure 1407265481921 Sync.Status DEBUG Status.login: error.login.reason.network => error.login.reason.network 1407265481921 Sync.Status DEBUG Status.service: error.login.failed => error.login.failed 1407265481921 Sync.BrowserIDManager INFO currentAuthState returning error.login.reason.network due to previous failure 1407265481921 Sync.Status DEBUG Status.login: error.login.reason.network => error.login.reason.network 1407265481921 Sync.Status DEBUG Status.service: error.login.failed => error.login.failed 1407265481942 Sync.BrowserIDManager INFO currentAuthState returning error.login.reason.network due to previous failure 1407265481942 Sync.Status DEBUG Status.login: error.login.reason.network => error.login.reason.network 1407265481942 Sync.Status DEBUG Status.service: error.login.failed => error.login.failed 1407265481942 Sync.BrowserIDManager INFO currentAuthState returning error.login.reason.network due to previous failure 1407265481943 Sync.Status DEBUG Status.login: error.login.reason.network => error.login.reason.network 1407265481943 Sync.Status DEBUG Status.service: error.login.failed => error.login.failed 1407265481957 Sync.ErrorHandler DEBUG Log cleanup threshold time: 1406401481957 1407265482019 Sync.ErrorHandler DEBUG No logs to clean up. 1407265482026 Sync.ErrorHandler DEBUG Log cleanup threshold time: 1406401482026 1407265482079 Sync.ErrorHandler DEBUG No logs to clean up. 1407265485062 Sync.BrowserIDManager INFO currentAuthState returning error.login.reason.network due to previous failure 1407265485063 Sync.Status DEBUG Status.login: error.login.reason.network => error.login.reason.network 1407265485063 Sync.Status DEBUG Status.service: error.login.failed => error.login.failed 1407265492773 Sync.BrowserIDManager INFO currentAuthState returning error.login.reason.network due to previous failure 1407265492774 Sync.Status DEBUG Status.login: error.login.reason.network => error.login.reason.network 1407265492774 Sync.Status DEBUG Status.service: error.login.failed => error.login.failed 1407265502512 Sync.BrowserIDManager INFO currentAuthState returning error.login.reason.network due to previous failure 1407265502512 Sync.Status DEBUG Status.login: error.login.reason.network => error.login.reason.network 1407265502512 Sync.Status DEBUG Status.service: error.login.failed => error.login.failed 1407265506696 Sync.BrowserIDManager INFO currentAuthState returning error.login.reason.network due to previous failure 1407265506696 Sync.Status DEBUG Status.login: error.login.reason.network => error.login.reason.network 1407265506697 Sync.Status DEBUG Status.service: error.login.failed => error.login.failed 1407265506698 Sync.ErrorHandler DEBUG Beginning user-triggered sync. 1407265506698 Sync.Service DEBUG User-Agent: Firefox/34.0a1 FxSync/1.36.0.20140804030205. 1407265506699 Sync.Service INFO Starting sync at 2014-08-05 21:05:06 1407265506699 Sync.Service DEBUG In sync: should login. 1407265506699 Sync.BrowserIDManager INFO currentAuthState returning error.login.reason.network due to previous failure 1407265506699 Sync.Status DEBUG Status.login: error.login.reason.network => error.login.reason.network 1407265506699 Sync.Status DEBUG Status.service: error.login.failed => error.login.failed 1407265506700 Sync.BrowserIDManager INFO currentAuthState returning error.login.reason.network due to previous failure 1407265506700 Sync.Status DEBUG Status.login: error.login.reason.network => error.login.reason.network 1407265506700 Sync.Status DEBUG Status.service: error.login.failed => error.login.failed 1407265506704 Sync.BrowserIDManager INFO Waiting for user to be verified. 1407265506704 FirefoxAccounts DEBUG already verified 1407265506704 Sync.BrowserIDManager INFO Starting fetch for key bundle. 1407265506705 Sync.BrowserIDManager INFO Fetching assertion and token from: https://token.services.mozilla.com/1.0/sync/1.5 1407265506705 FirefoxAccounts DEBUG already verified 1407265506706 Sync.BrowserIDManager ERROR Non-authentication error in _fetchTokenForUser: undefined 1407265506707 Sync.Status DEBUG Status.login: error.login.reason.network => error.login.reason.network 1407265506707 Sync.Status DEBUG Status.service: error.login.failed => error.login.failed 1407265506707 Sync.SyncScheduler DEBUG Clearing sync triggers and the global score. 1407265506707 Sync.SyncScheduler DEBUG Next sync in 3600000 ms. Also dozens of: 1407265815385 Sync.ErrorHandler DEBUG Flushing file log. 1407265815386 Sync.BrowserIDManager ERROR Background fetch for key bundle failed: No keyFetchToken 1407265815386 Sync.BrowserIDManager ERROR Could not authenticate: No keyFetchToken 1407265815387 Sync.SyncScheduler DEBUG Clearing sync triggers and the global score. 1407265815387 Sync.SyncScheduler DEBUG Next sync in 3600000 ms.

I have experienced this issue as well (also Nightly). As a workaround I disconnected my browser from FxA and reconnected and things have been working since.