[Graham]

Found the solution in this post:

https://buddypress.trac.wordpress.org/ticket/7704

Go to:

/wp-content/plugins/buddypress/bp-templates/bp-legacy/buddypress-functions.php

Around line 807, Find:

	// The template part can be overridden by the calling JS function.
	if ( ! empty( $_POST['template'] ) && 'groups/single/members' === $_POST['template'] && 'group_members' === $object ) {

Change “group_members” to just “members”:

	// The template part can be overridden by the calling JS function.
	if ( ! empty( $_POST['template'] ) && 'groups/single/members' === $_POST['template'] && 'members' === $object ) {

Fixed.

[Graham]

I deactivated all plugins and switched to a stock theme.

The problem remained.