Wordpress Screen Options, Help, Visual editor, Status, Visibility, Revisions, Published on and any toggled indicator don't work for posts

New to WordPress, I'm trying to debug the problem briefly described in the title of this question. None of these links works in our site when we edit a post. All work fine in any other components (Pages, Dashboard, etc.)

Edge Console shows up this warning:

HTML1521: Unexpected “/body>” or end of file. All open elements should be closed before the end of the document. post.php (765,1)

And I get these two errors in the debugger:

Also, by inspecting some of the elements, I show differences. For instance, regular functioning pages have a the containing the Screen Options button showing a style with a display property toggling between none and block when visibility changes by clicking the Screen Option button:

div class="metabox-prefs" id="screen-meta" style="display: none;"

and

div class="metabox-prefs" id="screen-meta" style="display: block;" 

This doesn't happen in posts, showing always:

div class="metabox-prefs" id="screen-meta"

I've not been able to find a relation between this situation and previous issues describing problems with the visibility of some of the affected elements in our installation. As only posts are in troubles here, it should be something related to the load of specific JS contents in such a WordPress element.

Please, have any of you seen something similar before? Could you point me in the direction of solving this issue?

The affected site is http://igfae.usc.es

Our installation:

• WordPress 5.1.1 running Foundry Child theme.
• Apache/2.4.6
• Running PHP version: 7.3.3
• Scientific Linux based on a Red Hat 4.8.5-28 box
• kernel 3.10.0-862.9.1.el7.x86_64 GNU/Linux

Thanks you very much for your help!

Ricardo