Error: Cound not find a ticket

We’ve been using RT 3.2.1 for many months now.

All of a sudden, when users reply to e-mail sent from RT with the subject
line intact, they get back a
message saying “Message not recorded. Could not find ticket with id #”

where # is the automatically generated ticket number in RT

These ticket numbers are valid and can be seen within RT itself.

What could be wrong? Where could we look for clues?
A N Ananth
Prism Microsystems, Inc Tel: 410.953.6776 x223