Content Filtering block page not shown correctly

The Content Filtering block page is not showed when a website is blocked, instead an error web page is shown by the browser.

The issue happens only when reaching HTTPS Websites and occurs when HTTPS Content Filtering is enabled and DPI-SSL is not implemented. 

HTTPS Content Filtering is IP based, and will not inspect the URL. While HTTP content filtering can perform redirects to enforce authentication or provide a block page, HTTPS filtered pages will be silently blocked.

This is an expected behavior if the DPI-SSL is not enabled.

The Block page always works if the traffic is HTTP while for HTTPS traffic it works only if the DPI-SSL is enabled otherwise the browser will show an error web page.

See also:

Where can I learn more about DPI-SSL? - https://www.sonicwall.com/support/knowledge-base/?sol_id=170503997881184
Which DPI-SSL migration process is best for my environment? - https://www.sonicwall.com/support/knowledge-base/?sol_id=170505555763507
How to configure Client DPI-SSL? - https://www.sonicwall.com/en-us/support/knowledge-base/?sol_id=170505885674291