AJAX menus not loading


I have recently upgraded OJS from from 2.4 to 3.3.014 (mostly due to a database that needed a lot of sanitizing and PHP7.4 not being available from my host for way too long) and finally resorted to doing the whole process offline on a XAMPP server.

Steps I took leading up to the issue

It took a while, but I got the whole thing working fine offline, and uploaded to my server over the weekend: everything went great! The journal ran like a charm! Clicked around a few links to make sure it was ok, great. I tried to install a new theme but got an error that Exec is not supported - fine, I can do that later. So I switched over the domain and went to bed satisfied.

On Monday, when I went back to the site, logged in to the backend: it was not fine. Well the site is there, My Queue is there, but the Archives list is empty (“No submissions found” and no badge with the number of articles in there) even though the archives do display on the frontend without problems.
More generally, none of the AJAX menus are loading, I just see the Loading… whirlygig which never materializes into a menu.
I’ve cleared the OJS cache, cleared the browser cache, etc, no change.

I also can’t download files - Chrome and Firefox just won’t do it, while Edge will display a warning that the file is malicious, which makes me think this may be a host error, but what kind?
We don’t have an SSL cert, could that be the issue?

One thing which may be fortuitous or meaningful: the System Information page is mangled.

What application are you using?

Additional information
Apache/PHP are not logging any errors - and my XAMPP server is still doing just fine on localhost with the exact same PHP version, files, and database.

Any help would be appreciated!

Update: I did a fresh install of the journal, and everything was perfectly functional until I assigned the domain - with the same results as above. So now I know where the issue is. I just don’t know what it is.