Add new Post page – Loads forever. PLEASE help!

Hi people, I am running this website:

https://teladoiolamerica.net

Everything else works perfectly EXCEPT for the “add new Post” page that sometimes stops working altogether and simply “loads forever” (the page stays blank, I can’t even get to the point where I can start typing).

This happens on a regular basis on periods that last hours!

This also happens for all our contributors (about 30) who can’t work for hours every day!

I tried disabling a few plugins that could be related to “adding new posts” like “Yet Another Related Post Plugin” and a Social Network Publisher… I don’t think disabling caching/akismet and others would change anything…

I use a child theme based on WP-Zoom.

Any ideas please?