/?random doesn't work

Could you try to deactivate all other plugins, one at a time, and let me know if you find a conflicting plugin? You’ll want to start with any plugins that may make changes to your site’s .htaccess file, or plugins creating or handling redirections.

Let me know what you find out.

I can’t deactivate plugins on my sites which are online and full of visitors.

But I have compared my several blogs, they all have the same plugins.
And error occurs on 2 blogs with the same theme.

Could it be the theme that causes the conflict ?

Actually it’s not an “error” but nothing happens when I go on /?random. It stays on homepage.

I can’t deactivate plugins on my sites which are online and full of visitors.

There is no magic bullet in these situations. This is just the start of basic troubleshooting and is a process of elimination deliberately designed to locate the root cause as quickly as possible by first removing the most obvious and common culprits via a series of (often) temporary steps.

Your plugins’ settings are saved in your database and should be retained for future use.

Curious thing.

I deactivated plugin “nrelate Most Popular” on blog A. Then random worked. I reactivated plugin and random still works.

I deactivated plugin “nrelate Most Popular” on blog B. Random still doesn’t work. Even after reactivaed plugin.

OK so I deactivated plugin https://www.ads-software.com/plugins/addthis-smart-layers/ and random redirection now works !

It is strange because this plugin is just some frontend display, not related with .htaccess or anything…

What could I do ?

I’m not familiar with this plugin, so I’m afraid I won’t be able to help, The plugin authors might be able to help you; they’ll know if their plugin can have an influence over redirects.
For reference, here is Jetpack’s Random Redirect module:
https://github.com/Automattic/jetpack/blob/2.7/modules/theme-tools/random-redirect.php

OK thank you Jeremy.