Cookie 35172 not found, dropping ack

Hobbit 4.2.0+allinone, CentOS 4.6 on x86_64...

I have users reporting that randomly, the hobbitd status on our server
is going yellow with the 'dropping ack' message.  A user just reported
to me that this happened about one hour after he had acknowledged a
yellow alert status.

Anyone have any experience troubleshooting where these come from?  I
suspect it may be another 64-bit issue, but am not yet sure.  Is the
cookies for alerts logged or recorded anywhere?  Thanks for any advice.
-Alan