I can't register in OpenSER using SIPp. SIPp versions tested 2.0.1 Windows (TLS), Latest from SVN on linux X-Lite registers w/o any problems.
In pcap file & messages you can see 2 regsitration attempts. first form x-lite (sucessfull) and second from SIPp.
This looks like OpenSER and SIPp have some differences in MD5 authentification.