Just an FYI to hopefully assist with troubleshooting.
 
Error 503 No healthy backends
No healthy backends
Guru Mediation:
Details: cache-sjc3120-SJC 1461134739 3560692205
Varnish cache server
 
04:46pm Australian Eastern Standard Time Wednesday 20th April. 
 
Windows 7 Professional PC
 
As my Subscriptions list was open from earlier in the day, I selected ‘Subscriptions’ to refresh it for new posts, receiving the above error message.  I selected 'Back' and clicked on this thread from my list, once again receiving the error.
 
I opened a new tab in Chrome for a new session of BYC and same error message.  Cleared the cached images and files in Chrome.  Error remained.
 
Opened an incognito session on Chrome.  Error remained when attempting to open BYC.
 
Internet Explorer.  Error remained.
 
Firefox.  Error remained.
 
Other web sites .. AOK
 
Finally, tried on a different computer in another room running Windows 10 [Chrome]. Error remained.
 
Still an issue at 05:13PM [Ducked outside to supervise the bedtime squabbles 
] and checked intermittently from then on.
 
Still an issue at: 05:21PM
 
Restarted Windows 7 Professional PC at 05:30PM, error remained on restart.
 
Rather than use by bookmark to BYC log in, I Googled ‘noisy chicken’ one of which I just happen to have  
 and picked a result which was on BYC; same error.
 
Still an issue at: 05:48PM; 06:02PM [on both Windows 7 and Windows 10 machines].
 
Checked again at 06:20PM and was able to log in without issue so came back on line some time between 06:02PM and 06:20PM
 
After all this you will probably tell me this was a scheduled server outage about which I missed the notification?  Or a larger issue, about which you were aware and not related to the 
intermittent Error 503 No healthy backends on which this thread is based