Hi Elliot,
I also migrated to WP3.6 a week ago. Today I had to adjust a slug of a post and I had the the same error. Also I deactivated all plugins and changed theme to Twenty Thirteen. Only when I active ACF and I go to post-types where I have ACF fields the error happens (my test is the slug-edit button and the screen-option tab).
I still testing further, but because I mention to Zartgesotten that only she has the problem I thought I inform you that she is not the only one.
Best regards,
Dennis
