> They have Interbase 6.0.0.627 and I use FB 1.3

There is no 1.3 release. That should be either 1.0.3
or 1.5, I guess. ;-)

If you see a process ibserver.exe running, then it
is Firebird 1.0.x (or InterBase ;-)). If you see
fbserver.exe, then you are running 1.5.x.

To get the entire version, simply go to the bin
directory of your Firebird / InterBase installation,
right-click the ibserver.exe or fbserver.exe and
move to the "Version" tab.

Inserting NULL into a field which is part of an
UNIQUE INDEX / CONSTRAINT won't work in < 1.5.


Best Regards,
Thomas Steinmaurer
LogManager Series - Logging/Auditing Suites supporting
InterBase, Firebird, Advantage Database and MS SQL Server
Upscene Productions
http://www.upscene.com