Blog page - Excerpts not working properly

I'm having some issues getting excerpts to display properly on my blog page.

https://www.extractedmagazine.com/blog/

The featured blog post throws a full blown error which displays like this:
"Warning: call_user_func_array() expects parameter 1 to be a valid callback, array must have exactly two members in .../wp-includes/class-wp-hook.php on line 287"

I've actually switched the blog to show full content because that error was displaying my server file structure on the page (which i edited out of the quote here)

In addition to that error, a number of posts don't show an excerpt at all, and a number of posts just showed full content.

I'm wondering if this is a problem with the way a number of my older posts are constructed (many of them written prior to Gutenberg).

Edit: I should say that the article which gives the error (the featured one), I did update with gutenberg, so it uses paragraph blocks and such.