[Registration failed. HTTP return code is 501] - Please, help :(

[DomainEmpire.com]

Since I've my new Pc (only some parts have been replaced to tell the truth ...), I'm unable to reinstall my HumanClick (humanclick.com - Direct live support for customers directly from our website ...).
At the end of the setup, when it asks me to enter my user/password, once I confirm this step, I get al alert message, the same displayed in the object ...

Does someone imagine what "Return code is 501" may stand for, please ?