A little research explains why it happened:
https://wordpress.org/support/topic/issues-with-wordpress-401-and-shortcodes
I guess we can wait until this plug-in is updated or revert back to WP 4.0 and live with the security hole.
replaced with https://wordpress.org/plugins/table-of-contents-plus/
no have time to fiddle with outdated or not supported plugins
I love TOC+ but it broke my Portfolio Slideshow plugin and never got fixed.
Perhaps we will get a fix to this one soon
Any update on this? I hate the plugin I have had to revert to.
This has been fixed now.. Thanks Ohad.