Due to Microsoft's security requirements for Lync phone software, a secure HTTPs password authenticated connection is needed in order to manage the Snom phone from the Web User Interface.
When you login to the Web user interface you are prompted to set the admin mode and HTTP server password for the phone. The phone will allow you to bypass this step and move to the configuration process of the phone.
However, once you create your first UC account on the phone, the Lync credentials will be used to secure the HTTP server access to the Web User interface. The phone does not authenticate the Lync/NTLM credentials against the Active Directory/Lync Front End/Edge before saving the settings, so if the username or password is entered wrong the phone will cache those credentials even if the SIP registration for the Lync account fails on the Front End or Edge.
If this happens the only way to recover the login password that was entered is by doing a factory reset of the phone.
You can avoid this from happening by configuring the HTTP Server user account prior to configuring the UC account on the phone.