Catchable fatal error: Object of class WP_Term could not be converted to string

Your problem isn’t line 780 of ../wp-includes/class-wp-query.php

So it’s a little bit hard to help because you’re not showing the actual code that’s at fault.

But you’re probably trying to reference a post category by its data object instead of using the integer id.

For example you might have something like this:

$mycategory=get_category_by_slug('a-category-slug');
$args = array( 'posts_per_page' => -1, 'category' => $mycategory);
$myposts = get_posts( $args );

But $mycategory is an object, not an integer value.

You would need to add ->term_id to get its integer value:

$mycategory=get_category_by_slug('a-category-slug');
$args = array( 'posts_per_page' => -1, 'category' => $mycategory->term_id);
$myposts = get_posts( $args );