User Sync PHP compatibility

Couldn't sync the single WP install getting the following error:
There was a connection problem. Please check the URL and Key of the Master site."

On debugging, I get the following error:

03-Aug-2017 03:40:22 UTC] PHP Fatal error: Uncaught Error: [] operator not supported for strings in /home/xxx/public_html/xxx/wp-content/plugins/user-sync/user-sync.php:878
Stack trace:
#0 /home/xxxx/public_html/xxx/wp-includes/class-wp-hook.php(298): User_Sync->user_sync_ajax_action('')
#1 /home/xxx/public_html/xxx/wp-includes/class-wp-hook.php(323): WP_Hook->apply_filters('', Array)
#2 /home/xxx/public_html/xxx/wp-includes/plugin.php(453): WP_Hook->do_action(Array)
#3 /home/xxx/public_html/xxx/wp-admin/admin-ajax.php(101): do_action('wp_ajax_nopriv_...')
#4 {main}
thrown in /home/xxx/public_html/xxx/wp-content/plugins/user-sync/user-sync.php on line 878