A. Please follow suggested fixes in FAQ Page not found.
B. Also, it's possible that your theme, like Karma, has created its own wp_pagenavi function that doesn't match the parameter profile of the similar named WP PageNavi plugin. The easy fix is to disable the theme from using it's own wp_pagenavi functionality and in turn use WordPress's built-in or the WP PageNavi plugin.
For Karma, the fix would be to comment out line 51 of `truethemes_framework/truethemes_framework_init.php`. Like so…
// if(!function_exists('wp_pagenavi')){require_once(TRUETHEMES_EXTENDED . '/wp-pagenavi.php');}
This is a solid example of when a theme is doing too much.
C. Don't use a regular post entry for paging through testimonials. That doesn't work as paging is only for the post itself.. Make your testimonials post a page.
D. If using Testimonials Premium, make sure "Prevent Duplicates?" isn't set if you're running testimonials listings and widgets on the same page and you don't have enough testimonial to support your set limits.