Categories not working

When I try to view a category list (e.g.: http://kailashtrekking.com/category/itineraries/pilgrimages/) I now get “no results were found” message, even though there are many posts within that category. The same is true for all the other category options that I’ve tried (via my site map at: http://kailashtrekking.com/about-us/site-map/)

This problem was realized recently. Perhaps it’s from the recent WP version? I’ve tried the remedies recommended in this thread: https://ww.wp.xz.cn/support/topic/categories-not-working-15, but that has not worked for me. I did try turning off all plugins. No change. And, I even switch the current them to TwentyFourteen theme. Still, no change.

If anyone can help here, that would be very much appreciated. Thank you in advance.