Please Help, Menu Editor does not work in subsites of multisite

Hi
I use WordPress multisite (version 5.5.3) with a main site (d-dar.com) and a subsite (d-dar.com/template).
I have a very frustrating issue with my subsite. I searched a lot on Google, and tried many solutions, but no luck.
When I go to the main site’s menu editor (Appearance> Menus) the menu editor works quite well. I can click on the headings in order to expand/collapse the headings (pages, posts, custom links, categories, etc.). I can add custom links, posts, etc. to my menu. It is OK.
However, when I go to a subsite, no matter to the current subsite or new fresh subsites, the menu editor does not work. I go to the menu editor (appearance> menus), and I see a few headings (pages, posts, custm links, categories, etc.). The first heading (Pages) is expanded by default, so I can add new pages to my menu; but the other headings (posts, custom links, categories) are all collapsed, and I cannot expand them, no matter how many times I click on them. That’s why I cannot add custom links, posts, categories, etc. to my menu. I can add pages only.
I also tried to go to Screen Options, with the hope to find a solution. I found no solution in Screen Options, but I found that, I could not expand/use the screen options of the subsite, while I could expand and use the screen options of the main site without any problem.
I am blind and I use screen reader software, so I asked a sighted person to check the page, but they could not expand the headings either. The mouse clicks work well in the menu-editor of the main site, but not in the menu-editor of the subsite.

I tried the following solutions, but none of them solved the issue:
Using various PCs, various operating systems, various internet browsers (Chrome, FFX, IE)
Deactivating all plugins
Activating the default theme (Twentytwenty)
Going to the network superadmin dashboard> updates> reinstall WordPress
Going to Public HTML folder and replacing .htaccess file with its previous versions.
Creating a new fresh subsite.
activating debug mode and logging mode in WordPress

Well, I have a few caching plugins, optimization plugins, security plugins, minification plugins and other plugins. I deactivated all plugins, but the problem still exists. I guess one of the plugins has damaged some JS/CSS file(s).
How can I find the cause of this problem?
How can I solve it? It is a critical issue. I cannot use my website without the menu editor.

• This topic was modified 3 years, 11 months ago by James Huff. Reason: moved to Networking WordPress since this is a multisite issue, deleted erroneous tags which were just a repeat of the content