> I am going to present the results of this here later.

And here we go:
After a backup/restore I could create and update the new column of the
table without any problems. But as I said before, this is not really a
solution.

More interesting facts: With version 1.0.3 on linux (CS) it seems to
be enough, if I split the script into two parts. So before updating
the column of the table I just reconnect to the database and that does
the magic trick. No backup/restore neccessary.

And now the workaround which really might help in THIS case: I just
moved the creating and updating of the new column to the beginning of
my script - and this seems to work on both: windows and linux.
Hopefully this also works on our customers side.

But still, I really would like to know what makes firebird behave this
way. So that in future I am aware of what I should not do in my scripts.

--Heiko