IB_Events and 'Access violation in module fbclient.dll'

In our application we get occassionally an
EAccessViolation with message 'Access violation at address 1002C389
in module 'fbclient.dll'' during shutdown.

The app is built with Delphi 7 / IBO4.6A and we use Firebird 1.5.3 as
Database (Superserver, TCP/IP Connection, the DB Server is on the
same PC as the client application)

In our Application we use more than one
TIB_Session/TIBConnection/TIB_Events components in order to use the
Firebird Event functionalty. The error occurs during shutdown of our
application when we unregister the events and then disconnect from
the DB. The error always occurs when isc_detach_database is called in
TIB_Connection. Typically the Delphi Debugger breaks inside ntdll.dll
and you can see an OutputDebugMessage
'HEAP: Free Heap block 15de2f8 modified at 15de380 after it was freed'
This occurs more than once untill the Access violation occurs and
then the Application "hangs".

The error occurs more often on older PC's and it occurred even in a
small test application !

Because we never had this Problem with Interbase 6 therefore i think
it might be a bug in fbclient.dll ???

Any help is greatly appreciated

Stefan Rust
Roche Instrument Center AG
Switzerland