:: Assuming Fb 1.5....

Yes sorry, remembered to mention this then forgot to type it. :)

:: 1. Did you generate a "compatibility client" gds32.dll using
:: instclient.exe and place it in the system32 directory?

I'm 99% sure. We had IB client installed, and when trying to fix this
problem we then installed FB 1.5 (with client) on the machine to try to fix
it.

:: 2. Does the client machine have a Services file entry to
:: listen for the host?

On the default port. But this port is not used if I specify the port - on
the developer machine the port is other than what this is and it connects
fine.

:: Did you follow the instructions for configuring an
:: alternative service port
:: in the Fb 1.5 release notes?

There are notes on this? :)

The problem is not the server though, other clients connect to it fine and
it is not on the end user machine. The problem is one client machine only.