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Nevermind, I tested it myself on https://instawp.com/ with PHP 7.4 and the error did not occur.
I will try to debug some further, but could you please let the developers know of the error message?
Maybe they have an idea what might cause such an error.

Hello,

Ok, good to hear you can reproduce (part of) the issue.
For a second I thought I was going insane 🙂

I did some further testing on my local host for the error message.
For the single site installation as well as for the multi-site installation I connected a user to a post and checked the post overview screen in the dashboard.

On the single site installation the error message does not occur and the relationship is shown correctly:

On the multi-site installation the error message does occur and the relationship is not shown correctly:

So it seems to be an issue with multi-site installations.
Can you confirm this?

Hello,

Ok, in that case it was coincidence that the error occurred only on servers with PHP 8.

I did some further testing on my local host.
I will try to replicate these steps on a live server, so I can give you access to it.

On my localhost I have:
PHP: Version 7.4.29
MYSQL: 10.4.24-MariaDB

I installed two clean WordPress instances: one single site and one multi-site.

The only plugins that are active are

• Metabox
• MB Relationships
• MB Relationships Debug (plugin that registers the relationship)

The debug plugin only registers one relationship:

add_action('mb_relationships_init', function () {
    MB_Relationships_API::register([
        'id'   => 'users_to_posts',
        'from' => [
            'object_type'   => 'user',
            'admin_column'  => [
                'title'         => 'Posts',
            ],
        ],
        'to'   => [
            'object_type'   => 'post',
            'post_type'     => 'post',
            'admin_column'  => [
                'title'         => 'Users',
            ],
        ]
    ]);
});

On both WordPress installations I get two Metaboxes on the post edit screen:

Also I get the following error message:

WordPress database error You have an error in your SQL syntax; check the manual that corresponds to your MariaDB server version for the right syntax to use near 'mbr.order_to' at line 6 for query \n\t\t\t\tSELECT wp_users.ID , mbr.to\n\t\t\t\tFROM wp_users INNER JOIN wp_usermeta ON ( wp_users.ID = wp_usermeta.user_id ) INNER JOIN wp_2_mb_relationships AS mbr ON (mbr.from = wp_users.ID AND mbr.type = 'users_to_posts' AND mbr.to IN (1)) \n\t\t\t\tWHERE 1=1 AND ( \n wp_usermeta.meta_key = 'wp_2_capabilities'\n)\n\t\t\t\tmbr.order_to\n\t\t\t\t\n\t\t\t made by WP_List_Table->display, WP_List_Table->display_rows_or_placeholder, WP_Posts_List_Table->display_rows, WP_Posts_List_Table->_display_rows, WP_Posts_List_Table->single_row, WP_List_Table->single_row_columns, WP_Posts_List_Table->column_default, do_action('manage_post_posts_custom_column'), WP_Hook->do_action, WP_Hook->apply_filters, MBR_Admin_Columns->post_to_column_data, MBR_Admin_Columns->get_column_data, MBR_Admin_Columns->get_user_items, get_users, WP_User_Query->__construct, WP_User_Query->query, referer: http://localhost/wp-dev-2/mb-relationship-debug/wp-admin/post.php?post=1&action=edit

I don't like sharing credentials unless absolutely necessary.
Have you tried to replicate the issue first? And it did not occur?

Ok, I will do that.

Thanks for the reply.

p.s. the img I included does not load (it's a google drive link), however if you open the link directly in the browser it will load.