403 forbidden error when moving entries in structure We're running on fortrabbit.

I cannot sort entries in a structure without getting:

403 index.php?p=admin/actions/structures/move-element

Unauthorized
User is not authorized to perform this action


This is the only user and it's a super user / admin.
You've probably got a multi-node/web server setup in Fortrabbit, but haven't setup memcache for PHP session storage. They have an article on how to do it here: https://help.fortrabbit.com/craft-3-tune#toc-cache-amp-session-on-the-professional-stack

Moving items in a Structure in Craft requires a working PHP session.