XMLRPC library does not reset xmlrpc error code before a new xmlrpc call is requested

Is this necessary? It seems to me you can tell whether the request succeeded by checking if the return value of xmlrpc() !== FALSE. Only then do you need to call xmlrpc_errno() and friends to find out more.

Indeed. As the docs say, for a simple call, check the return value of xmlrpc first and only on error turn to xmlrpc_error. On system.multicall you do not even need to call xmlrpc_error as the error objects are among the results (check the result for being an object and ->is_error TRUE).

What happens if I expect a boolean value from my xmlrpc call? How can I distinguish between the FALSE from an error and a FALSE I got from the remote server.

i've got the same problem as wyssp

just suggesting, if you are the one who also did the function on the server side, just let it return an int instead of a boolean so you can safely check for === false for error