I've  M2.10 , Varnish , Redis and Nginx intalled and using Php7.08 with installed after doing a lot of research I wasn't able to find a solution for this. I was moving around attributes from groups to un-assigned group in Magento 2.10, I notice I was having issue with Varnish and the error was Varnish 503 Backend Fetch Failed. The problem was in "clean_cache_by_tags" function in Magento. I think it's an error clearing the cache in php7 & magento2. Quick fix is to turn off block_html cache. Everything works in Php 5.6x but in PHP7.08 its causing a lot of problem. Should I downgrade or stay upgraded is the question ? After installing php5.6 it appears everything seems to be working fine

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