Phex should accept this handshake if there are peer slots open.
Other clients will most likely reject it since Phex is one of the rare clients left that still support peer connections and handshakes without the ultrapeer headers. I think we even support incoming 0.4 handshakes...
Though the peer and 0.4 handshake is not tested well, since its not used anymore. It could be that your connection is not very reliable.
BTW: Phex has no 500 return code.
Gregor
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