UDP problems are generally caused by firewall configuration. By default Aspera products use both TCP port 22 and UDP port 33001 for transfer tasks. When you see UDP-related error message, first make sure UDP port 33001 is opened for both inbound and outbound traffic. On Windows and Macintosh, open a range of UDP ports (for example 33001-33010) if you plan to run multiple concurrent connections.

The other scenario is UDP port reutilization on Linux and Solaris. If a client computer starts a transfer to a server, it creates an ascp or scp.aspera binary on the server, which receives the client's UDP packets. However if the client computer goes to sleep or the network goes down, the server side ascp or scp.aspera may cause the server side process not toterminate. If the same client computer starts a second transfer, it creates a second ascp or scp.aspera binary on the server, as expected This second server side ascp or scp.aspera process will not get any UDP data while the first ascp or scp.aspera process is still running.

The solution is to terminate the scp.aspera or ascp on the server manually, then reinitiate the transfer.