Hi!
Happy new years to you too!
” .. if I want to add it to all other blog posts it disables the whole sites.”
Are you using the same method as on the working post (using a shortcode), or the internal plugin option to display it on all posts?
The error message unfortunately refers to another plugin, but I’m almost sure it’s related posts pro still causing the problem. Recently there was an issue discovered which concerns the last update (1.5) where the singular posts and the main query are not correctly recognized and the plugin hook might get executed multiple times, especially if a page builder plugin is involved.
The fix is a bit complicated to explain line by line, so I will rather link the fixed file here. Just replace the content of the wp-content\plugins\related-posts-pro\includes\hooks.php file on your web server with this one: https://gist.githubusercontent.com/anag0/fa0c44d214506daf45e65e132a05accc/raw/4b28d20ebf5079ed48a344356a8a0fce59229680/hooks.php
Make sure to have a back-up copy of everything just in case something goes wrong. If the issue was related to the known problem, this should hopefully solve it.
Best,Ernest Marcinko
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