Express Talk with AXON

I've set up AXON at our server, using a SIP-account that is working fine if using it directly with my SIP-Client (former X-Lite, now Express Talk).

But if I want to establish a communication with AXON, Express Talk tells me:

Error returned : 407 Proxy Authentication Required

Check username or password for the proxy server.

I've exactly entered my User/PWD as given in the Setup Details at the AXON-console - or do I need a real domain-user to connect to the proxy?

Thanks in advance!