Explode() expects a string

Let’s start from the top:

1. $current = get_the_category();

   This returns an array of objects, one object for each category.

2. $current_id= $current[0] ->cat_ID;

   This returns an object, the first object in the previous array

3. $categs_list = get_category_parents($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?)

4. $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.