Invalid transaction handle when idle > 10 min

[StephanM]

Thanks for your suggestion Alexandre.
Those settings were correct.
TIWServerController:
SessionTimeout = 30 ;
App Pool:
Limit Interval = 35
Idle Time-out = 35
Failure Interval = 10
Recycling, Regular Interval = 1440 (was 1740)

But it's not the application that stops running, only its DB connection that is lost.
I don't think it's a problem with the code since the same exe does not have this issue on the old server.
And I'm not aware of a Firebird DB server having a connection timeout.

Any ideas of what to check next?
I'll report back if I find anything on my end.