Hello, Jurgen!

I may suppose that it is related with different versions of gds32.dll (its a
former of fbclient.dll) and your server.
Local protocol is different in old FB/IB and in the Firebird 1.5, but TCP is
still compatible,
that is why you are able to perform backup

The last computer where the TCP connect doesn't work probably has problems
with TCP/IP settings.

And, in FB1.5 classic (AFAIK) there is no local protocol at all.

Sincerely yours,
Alexey Kovyazin
www.ibsurgeon.com/interbase/firebird/error/commonerrors.html