Redis error in incognito / private browsers

[RossCC]

Hi folks

In some browsers, in their private browsing modes, sometimes, we are getting this error:

Fatal error: Uncaught RedisException: read error on connection in /var/www/vhosts/myshop.co.uk/httpdocs/vendor/colinmollenhour/credis/Client.php:1130 Stack trace: #0 /var/www/vhosts/myshop.co.uk/httpdocs/vendor/colinmollenhour/credis/Client.php(1130): Redis->select(1) #1 /var/www/vhosts/myshop.co.uk/httpdocs/vendor/colinmollenhour/credis/Client.php(619): Credis_Client->__call('select', Array) #2 /var/www/vhosts/myshop.co.uk/httpdocs/vendor/colinmollenhour/cache-backend-redis/Cm/Cache/Backend/Redis.php(369): Credis_Client->select(1) #3 /var/www/vhosts/myshop.co.uk/httpdocs/vendor/colinmollenhour/cache-backend-redis/Cm/Cache/Backend/Redis.php(242): Cm_Cache_Backend_Redis->_applyClientOptions(Object(Credis_Client)) #4 /var/www/vhosts/myshop.co.uk/httpdocs/vendor/magento/zendframework1/library/Zend/Cache.php(153): Cm_Cache_Backend_Redis->__construct(Array) #5 /var/www/vhosts/myshop.co.uk/httpdocs/vendor/magento/zendframework1/library/Zend/Cache.php(94): Zend_Cache in /var/www/vhosts/myshop.co.uk/httpdocs/vendor/colinmollenhour/credis/Client.php on line 1148

Presumaby Redis trying to do something it can't, but a nicer error would be better.  PHP error display is actually disabled, so this shouldn't be showing at all.

Magento CE 2.2.6, was working prior to upgrade to this version a few weeks ago.

Any ideas?

[RossCC]

For the benefit of anyone who has this - in out case they were occasional genuine Redis timeout errors, which for some reason NGINX was caching, ie caching the error page.

So subsequent visit by other users got served the error page from the cache. 

Now trying to think how this happened and how to stop it!