SOLVED - Changed code to use explicit transactions and the problem went
away, so I guess implicit transactions was "keeping the transaction opened"
for a few extra milliseconds... while the other "process / thread / or ODBC
object" was trying to read the data added by the last transaction, and as it
was not commited yet, the data was not visible. Many many years ago, like 15
years ago, a friend who advised me to use Firebird told me "dont use
explicit transations with Firebird; you'll be better of leaving Firebird
manage it implicit..."; it did not sound correct then.... and it came back
to bite me now...

Cheers,
Fabian