> We're having a problem since we upgraded to Firebird 2.1.2 (and Firebird
> 2.1.3) that at one customer it keeps on crashing on a daily basis mostly
> with error: "page nnn is of wrong type (expected 5, found 4)".

It is NOT a crash. It is bugcheck. And it is really different things.

..

> The error is reported at random in any of the 3 databases that are on
> the system, including security2.fdb.

Are you sure HW is ok ? How big load on security2.fdb ? Any antivirus\
firewall\backup SW is installed at this customer machine ?

> Once problem started incoming connection using TCP/IP are denied and
> only local protocol connections can get through.

It is even more strange as remote server layer have no relation with the
engine in this regard.

> Real database corruption has never happened and a simple stop/restart
> firebird is enough to clear the problems away.

Sounds better

> I have the impression that the real problem is the same one that causes
> http://tracker.firebirdsql.org/browse/CORE-2616 which generates a "page
> nnn is of wrong type (expected 7, found 5)"
>
>
>
> Any suggestions in helping us narrow this problem down are greatly
> appreciated.
>
>
>
> We have managed to get the system stable enough that it only crashes
> late in the day, usually after 7PM, however, it used to crash more than
> once a day.

As you said the issue occurs at only one customer i guess his HW or
SW have something which make FB work badly. As you said you changes
different VMs and OSes, i guess this may be HW or host OS related.

Regards,
Vlad

PS Try to reproduce the issue. As you saw CORE-2616 was fixed very soon
after case was implemented.