Sync encountered an error while synchronising - On every launch!

[Zoom7000]

I'm running Windows 10 (10130) and I installed on to my main PC from a clean build. Since installing Firefox, I get the following error on every single launch:

 

"Sync encountered an error while synchronising: Incorrect account name or password. Sync will automatically retry this action."

 

If I put my sync password in, it will carry on happily, but it seems to want to do this on every single launch. I have tried deleting the signedInUser.json as highlighted in one post on Mozilla support. But that hasn't worked.

 

Here is an extract from my about:sync-log file, if anyone can make sense of it

1435020353545	Sync.Service	INFO	Loading Weave 1.40.0
1435020353548	Sync.Engine.Clients	DEBUG	Engine initialized
1435020353550	Sync.Engine.Clients	DEBUG	Resetting clients last sync time
1435020353554	Sync.Engine.Bookmarks	DEBUG	Engine initialized
1435020353556	Sync.Engine.Forms	DEBUG	Engine initialized
1435020353559	Sync.Engine.History	DEBUG	Engine initialized
1435020353561	Sync.Engine.Passwords	DEBUG	Engine initialized
1435020353564	Sync.Engine.Prefs	DEBUG	Engine initialized
1435020353566	Sync.Engine.Tabs	DEBUG	Engine initialized
1435020353566	Sync.Engine.Tabs	DEBUG	Resetting tabs last sync time
1435020353569	Sync.Engine.Addons	DEBUG	Engine initialized
1435020353570	Sync.Service	INFO	Mozilla/5.0 (Windows NT 10.0; WOW64; rv:38.0) Gecko/20100101 Firefox/38.0
1435020353572	Sync.SyncScheduler	DEBUG	Clearing sync triggers and the global score.
1435020353573	Sync.Status	DEBUG	Status.service: success.status_ok => success.status_ok
1435020353573	Sync.Service	DEBUG	Caching URLs under storage user base: [url]https://sync-204-us-west-2.sync.services.mozilla.com/1.5/20492031/[/url]
1435020353573	Sync.Status	DEBUG	Status.service: success.status_ok => success.status_ok
1435020353573	Sync.AddonsReconciler	INFO	Registering as Add-on Manager listener.
1435020353574	Sync.AddonsReconciler	DEBUG	Adding change listener.
1435020353630	Sync.Engine.AdblockPlus	DEBUG	Engine initialized
1435020353630	Sync.Status	DEBUG	Status.service: success.status_ok => success.status_ok
1435020353640	FirefoxAccounts	INFO	Can't find the rest of the credentials in the login manager
1435020357632	Sync.Status	DEBUG	Status.service: success.status_ok => success.status_ok
1435020357633	Sync.Service	DEBUG	User-Agent: Firefox/38.0.5 FxSync/1.40.0.20150525141253.
1435020357633	Sync.Service	INFO	Starting sync at 2015-06-23 01:45:57
1435020357633	Sync.Service	DEBUG	In sync: should login.
1435020357634	Sync.Status	DEBUG	Status.service: success.status_ok => success.status_ok
1435020357634	Sync.Status	DEBUG	Status.service: success.status_ok => success.status_ok
1435020357636	FirefoxAccounts	INFO	Can't find the rest of the credentials in the login manager
1435020357637	Sync.BrowserIDManager	INFO	Waiting for user to be verified.
1435020357637	FirefoxAccounts	DEBUG	already verified
1435020357638	Sync.BrowserIDManager	INFO	Starting fetch for key bundle.
1435020357638	Sync.BrowserIDManager	INFO	Unable to fetch keys (master-password locked?), so aborting token fetch
1435020357638	Sync.BrowserIDManager	INFO	Background fetch for key bundle done
1435020357638	Sync.Status	DEBUG	Status.login: success.login => success.login
1435020357638	Sync.Status	DEBUG	Status.service: success.status_ok => success.status_ok
1435020357638	Sync.Service	INFO	Logging in the user.
1435020357638	Sync.Service	DEBUG	Caching URLs under storage user base: [url]https://sync-204-us-west-2.sync.services.mozilla.com/1.5/20492031/[/url]
1435020357640	FirefoxAccounts	INFO	Can't find the rest of the credentials in the login manager
1435020357640	Sync.BrowserIDManager	DEBUG	unlockAndVerifyAuthState re-fetched credentials and is returning: error.login.reason.account
1435020357640	Sync.Service	DEBUG	Fetching unlocked auth state returned error.login.reason.account
1435020357640	Sync.Status	DEBUG	Status.login: success.login => error.login.reason.account
1435020357640	Sync.Status	DEBUG	Status.service: success.status_ok => error.login.failed
1435020357641	Sync.SyncScheduler	DEBUG	Clearing sync triggers and the global score.

Any ideas?