Update based on the comment:

I think your best shot is to use the PHP output buffers. So you don’t have to repeat the mark up and you can implement what you want with a single while loop.

Example:

if (have_rows('repeater')): 

    $current_user = wp_get_current_user();
    $adminOnlyContent="";
    $nonAdminContent="";

    while( have_rows('repeater') ): the_row();

        $show_only = get_sub_field('show_only_to_logged_in_admins');

        ob_start();

        echo '
';
        echo '
';
        the_sub_field('title');
        echo '
';

        if ((user_can( $current_user, 'administrator' )) & ($show_only == 1)) {
            $adminOnlyContent .= ob_get_clean();
        } else {
            $nonAdminContent .= ob_get_clean();
        }

    endwhile;

    echo '
';

    echo $adminOnlyContent;

    echo $nonAdminContent;

    echo '
';

endif;

Another option is implement a separate function which will return the markup.

Original Answer:

You cannot use the_row() twice within a loop. So you cannot use the nested loops here. But there are several ways to achieve what you want.

Here is one option: Store the content for both groups of fields in variables and echo them after the while loop.

$other_rows = [];

if (have_rows('repeater')): 

    $adminFields="";
    $nonAdminFields="";

    $current_user = wp_get_current_user();

    while( have_rows('repeater') ): $row = the_row();

        $show_only = get_sub_field('show_only_to_logged_in_admins');

        if ((user_can( $current_user, 'administrator' )) & ($show_only == 1)) {

            $adminFields .= '
';
            $adminFields .= the_sub_field('title');
            $adminFields .= '
';

        } else {
            $nonAdminFields .= '
';
            $nonAdminFields .= the_sub_field('title');
            $nonAdminFields .= '
';
        }

    endwhile;

    echo '
';

    echo $adminFields;
    echo $nonAdminFields;

    echo '
';

endif;

Alternate solution:

The other options may require at least two loops. You need to use the loops one after the other instead of nested loops.

...
    $current_user = wp_get_current_user();

    while( have_rows('repeater') ): $row = the_row();

        $show_only = get_sub_field('show_only_to_logged_in_admins');
        if ((user_can( $current_user, 'administrator' )) && ($show_only == 1)) {

            // Display content

        }

    endwhile;

    while( have_rows('repeater') ): $row = the_row();

        $show_only = get_sub_field('show_only_to_logged_in_admins');
        if (! (user_can( $current_user, 'administrator' )) || ($show_only != 1)) {

            // Display content

        }

    endwhile;
...

Alternate Solution 2

Here is another technique: Get all the repeater rows and segregate them to admin and non-admin rows. Then you can loop over them. Here also you need to use two loops.

// This is just a pseudo-code but you get the idea
$rows = get_field('repeater');
$current_user = wp_get_current_user();

$adminOnlyRows = array_filter($rows, function ($row) {
    $show_only = $row['show_only_to_logged_in_admins'];
    return (user_can( $current_user, 'administrator' )) && ($show_only == 1);
});

$nonAdminOnlyRows = array_filter($rows, function ($row) {
    $show_only = $row['show_only_to_logged_in_admins'];
    return ! (user_can( $current_user, 'administrator' )) || ($show_only != 1);
});

echo '
';
    foreach ($adminOnlyRows as $row) {
        // Display content
    }

    foreach ($nonAdminOnlyRows as $row) {
        // Display content
    }
echo '
';