SDR failed to connect to backup server

Hi, our backup server is BE 2014 SP1 on Windows Server 2008 R2 SP1. We are trying to restore full system of some clients by using SDR. However, after we boot the client to recover with the SDR boot CD, when it asks for connections to the backup server, although we put in the correct username and password, it always says it cannot connect to the backup server. Both the backup server and clients are in WORKGROUP. There is no domain in this network. 

Have anyone encountered this issue before? Any advice? Thanks in advance.