At the server side, there are left unclosed attachments.
These conenctions are closed at OS level (netstat -a -n does not list them).
Firebird still lists them in MON$ATTACHMENTS.
In Firebird.conf, I have set up:
ConnectionTimeout = 300
DummyPacketInterval = 120
Unfortunatelly, this does not help and connections are open, until Firebird is restarted.