[Plugin: WP Super Cache] Test Cache Option Fails

I’ve installed WP Super Cache in my site. The frontend is loading perfectly with footer timestamps in the source as follows:

<!-- Dynamic page generated in 2.721 seconds. -->
<!-- Cached page generated by WP-Super-Cache on 2016-05-02 08:54:41 -->

<!-- Dynamic Super Cache -->
<!-- Compression = gzip -->

However, when I click on ‘Test Cache’ I sometimes get DB connection error (only appears while test caching) or I get a blank page saying connection error. What am I doing wrong?

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I have an additional query: Although gzip is installed Webpagetest.org or Google Page Speed says gzip compression is not enabled and I’m getting a score of around 30 in both desktop and mobile which is complete disastor!

Please help.