I’m seeing this same issue on one of my older servers. Still trying to track it down, but I did find a clue…. In my web server’s logs, I find:
[client 1.2.3.4] PHP Warning: http_build_query() expects at most 2 parameters, 3 given in /path/to/wp-includes/http.php on line 251, referer: https://www.mysite.com/wp-admin/plugin-install.php
Line 251 is exactly the same on both the working servers and the non-working one…
