I got the same problem, my plugins gallery remains empty, but PHP logs are showing this error:
Could not connect to 10.20.0.21 (501; 5), referer: https://journals.reduc.edu.cu/be/management/settings/website
[Sat Nov 14 16:46:47.226610 2020] [php7:warn] [pid 160] [client 10.20.1.33:11599] PHP Warning: DOMDocument::loadXML(): Empty string supplied as input in /data/services/www/journals/lib/pkp/classes/plugins/PluginGalleryDAO.inc.php on line 57, referer: https://journals.reduc.edu.cu/be/management/settings/website
This server is a fresh new installed version of OJS 3.2.1.1, in the log as you see I have this 501 error,
Regards
Eduardo