Yess!
Today I found out where the problem was.
It seems windows Vista will 'remember' the fax-accounts of shared fax-printers despite removing it from the printers section in control panel.
When you install a new fax shared printer, the account in Vista Fax and Scan is nou automatically updated to the new fax-server.
Therefore the client still tried to connect to the 'old' fax-server in our network, which was off course unavailable (thus RPC server error,etc.)
To change the fax account open Fax and Scan in the Vista client.
Click 'FAX' in the left lower pane, click tools in the menu and then choose fax'-account.
In this window you can enter the name of the fax-server, NOT the fax share .
The example dialog box shows \\myserver, but this is also WRONG!
The only thing you need to enter here is "myservername"(without the quotes)
Everything works perfectly now.