It's possible you've reset the crop option during the placement of another PDF or .ai file and it's set to somethign that doesn't exist in the PDF you are trying to place (Bleed box, or Trim Box are likely candidates). Try showing the options when you place the file and check the crop options. It should be sticky for both PDF and .ai until changed again.
too strange: I ctrl-d-placed with showing the options, DID CHANGE NOTING, and it would place the pdf anyway. Now I can drag-drop-place again - too strange, but anyhow: thanks for the advice!!