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I was getting same error that 7107digital described here: https://wordpress.org/support/topic/causes-error-on-update/
Notice: register_uninstall_hook was called incorrectly. Only a static class method or function can be used in an uninstall hook. Please see Debugging in WordPress for more information. (This message was added in version 3.1.0.) in /**********/public_html/wp-includes/functions.php on line 4146
I managed to resolve this by editing the hc-custom-wp-admin-url.php page for the plugin and replacing the line:
register_uninstall_hook( __FILE__, array( $hc_custom_wpadmin_url, 'uninstall_plugin' ) );
with
register_uninstall_hook( __FILE__, 'uninstall_plugin' );
as per: https://exceptionshub.com/notice-register_uninstall_hook-was-called-incorrectly.html
Haven’t tested uninstall the plugin, but this removed the error 🙂