Invalid folder
Folder-scoped API (e.g. comments) returned 400 Invalid folder. for a bad/missing folderId.
Why this happens
Empty id, oversized id, or corrupted link. Distinct from Folder not found (valid format, no row).
Diagnose and fix
1. Open the folder from My Files / public folder UI (do not craft ids).
2. Refresh if the URL is truncated.
3. See Public folders / You cannot comment on this folder for comment policy issues.
4. See Folders.