503 Backend fetch failed - Varnish Cache server Laravel

I have a Laravel web app and randomly in the admin I get the following error:

Error 503 backend fetch failed 
Guru Meditation:
XID: 499820
varnish cache server

I get this error even on the default Laravel login auth when I click login sometimes randomly it could throw this 503 error if I press back the session has started and I'm logged in.

I've had a look online but I'm not sure where to start to find out how to resolve this issue. many of the existing question on Stack seem to relate specifically to Magento CMS.

via Chebli Mohamed