Hi FunkenTelechy,
There might be an incorrect orphan record in VC_REQ table pointing to this REQ_ID; with help from your Quality Center Administrator or Data Base Administrator ; you need to get this orphan record deleted.
Nice to hear that there are no records in LOCKS table that are holding this requirement.
With the help of Site Administrator if it is possible from their end take the entire project backup( database/repository ); verifythis project and then repairthe same project for fixing this issue.
I have searched in google with the fixed string "You cannot check in.The entity may be in use by another user or may already be checked in."
Noticed that few other users also faced similar issue , please go through their notes and suggestions as well.
Note: Opinion's expressed are mine alone! No one else is associated with this.
Regards,
Srihari