Yes it is the Table of Contents Plugin, obsidian-plugin-toc. It used to work. And it still works on notes that have a TOC from the past, see how there are two TOCs in the screenshot. Won’t work on qa new note.
I can’t capture the error message it is only for a few seconds. When I use the hotkey I get this message (a black box on the right of the note): “No headings below cursor matched settings (min:2) (max:6)”