> > Old server is Windows 2000, SP 4 - Firebird 1.5.1.4481
> > New server is Windows 2003, SP 2 64bit - Firebird 2.5rc2 64bit
>
> I still have the problem and tried different installs. If I use the
> 2.5rc2 32 bit on my 64 bit Windows server I can open the old files.
>
> If I use the 2.5rc2 64 bit edition, opening old files is not possible.
> It only works with backup restore.
>
> I noticed the same problem with FB 2.1.
>
> So FDB files from 32bit and 64 bit edtions are not compatible.

The first ODS which supports 64-bit edition of Firebrid is ODS 11.1

So, you can work with database in ODS11.1 and higher using both 32 and 64 bit editions of Firebird (2.1 and higher), and
you can't work with database in ODS 11.0 and lower using x64 bit edition of Firebird.

Regards,
Vlad