“The REST API did not process the context query parameter correctly.” Custom theme conflicting with Media Library

I’m having the “The REST API did not process the context query parameter correctly.” error message on a custom theme, the media library isn’t working correctly. I do have shortcodes on my function.php file, I double checked with the solution posted here and I have done the output buffering correctly, so I’m lost here trying to find what I did wrong. Thanks in advance!