Destination folder does not exist
Move/bulk-move returned 400 because the destination folder is gone or invalid.
Why this happens
Stale UI after delete, wrong folder id, or folder not owned by you. Related: Folder not found / Invalid destination folder (Phase 59).
Diagnose and fix
1. Refresh My Files library (browse, search, sort); pick a current folder.
2. Recreate the folder if it was deleted.
3. Retry the move.
4. See Folders.