Whenever I login to magento backend it shows invalidated Block HTML Cache. This is happening even without making a change to the website or database. Why is that and how to fix it? I am using Magento ver. 18.104.22.168
Your question is already answered in this thread.
I am quoting the vital point below:
In Magento, whenever you make changes to products, static blocks, etc, it recognizes that the data in the database is no longer the same as what it has in the cache. Unfortunately, Magento doesn't realize what cache data is different, just that something is different.
You can setup a cron job to refresh cache. Sample code is also available with that thread.