Ann,
Thanks for your answer. Your description of the history is exact.
I installed firebird from debian. A know they do their own
manteinance, so I left my questions to the manteiner, but neither he
can find the solution or discover the cause.

The scenario is the following:

Old server:
Pentium III 500 MHz, 256 KBytes RAM
Fedora Core 2, Linux kernel 2.6.5-1.358
FirebirdSS-1.5.2.4731-0

New server:
Penium IV 3.0 GHz, 1 GByte RAM
Debian etch, Linux kernel 2.6.18-3-686 #1 SMP
firebird2-super-server 1.5.3.4870-12

Thanks again for your support.
Aldo