>Yes, I have run an Expire Inventory, but it does not say anything about
>backupsets.  Only backups, archives, database y recovery plan.

Maria - Thanks for verifying.  That's most peculiar, making it seem like
        something got left out of development of your 3.7.3 server.
I just thought to have a look at the README for the HP/UX 3.7.4 server
(ftp://service.boulder.ibm.com/storage/tivoli-storage-management/maintenance/server/v3r7/HP-UX/LATEST/TSM3740SRV_HPUX.README)
and, sure enough, there is:  IC27123  BACKUPSETS DON'T EXPIRE PROPERLY !
That may well solve your problem, without too much fuss.  Have a look.
And if you apply that, please report back to the List on how it does.
  Richard Sims, BU