"invalid request handle" opening IBQuery

I have a set of 3 apps which form a 24x7 monitoring suite for a system: 2
back-end apps and a front-end GUI app, all connected to an IB database.  The
GUI app is triggered by a TIBEvent component so that it does not have to
poll the database for changes.  The IBEvent component starts a timer, and
when that timer expires the I refresh the stats displayed to the operator,
using a TIBQuery.  The timer is there to prevent a slew of queries from
"thrashing" the database every time one little thing changes.

Last night I left all three apps running on my machine overnight, and when I
came in this morning, there was an "invalid request handle" error on my
screen, which had occurred when trying to re-open the query to refresh the
values on the GUI.

I am using D6ent sp2, IBX 6.03 and IB "ib_wi_os_tIB6_0_2_0.exe".

Any thoughts on what could be causing this error?

Thanks in advance,