Let’s start from the top:
-
$current = get_the_category();
This returns an array of objects, one object for each category.
-
$current_id= $current[0] ->cat_ID;
This returns an object, the first object in the previous array
-
$categs_list = get_category_parents($current_id);
This returns a string, but only if the current category has parents; otherwise, it returns whatever was passed to it: which was an object
(See where the problem is yet?)
-
$pieces = explode("https://wordpress.stackexchange.com/", $categs_list);
This expects a string, but is returning an error, because it is being provided an object.
So, Conclusion:
You are calling this code in a Post that is in a category that doesn’t have any parents. You need to write some fallback code to account for this occurrence.