At the time of material movement was there any inspection type activated in material master? You can check in MM03 with display changes in material master to check if someone has deactivated it after the movement.
As I read further, you are getting error while doing 321 through MB1B. That drives me to conclusion that an inspection lot must be open in system. Could you please check in QA32 (by choosing option select all inspection lots with wide date range) if there is any lot with status SPRQ or SPST? If you you can post the stock against it.
If you still do not find any such inspection lot in system, I would go with Arijit to implement 48815. Let us know if this resolves your issue.
Anand