OK I took a closer look at the error message, and it does appear the phone is encoding the user wrong. Mobilesync picks up user credential from HTTP basic auth. In the HTTP request the client is supposed to send a header like this:
Authorization: Basic <credential>
where <credential> is base64 encoded user auth data. The auth data can be any of the following:
user
ass
user@domain
ass
domain\user
ass
domain\user@domain
ass
It looks like when the phone encodes the auth data, it mistakenly used a forward slash '/' instead of '\' to separate domain from user.
If the phone allows omitting domain, you can try to leave the domain empty and make username in the form of user@domain. Hope this will workaround the phone problem. If not, you'll need a fix in the phone software.