Ok I found pfff...
My "Parent" had as parent, "Parent". So I removed it thanks to http://.../admin/manage/categories/* urls...
Would be great to be forbidden or something logged 😉
after migrating data from another forum engine to nodeBB at Maxon - @Jay-Moonah - we found that the "In categories" field in Advanced Search is empty.
We've never experienced it before, and I wonder if it's a theme issue or a data migration issue.
PS. dbsearch indexes have been successfully recreated.
Sounds like this might be an import issue that can be fixed on the next run, could you please include this in a direct email?