Unified Self-Service (USS) not using NTLM (SSO) 14.1 and later
Service Catalog (SC) using NTLM authentication 14.1 and later
Service Catalog is using SSO so when trying to log into USS (not using SSO), this will call Catalog which expects for the credentials to be provided by SSO, causing the issue to occur.
1) Log into Service Catalog as an Administrator
2) Administration > Configuration > Single Sign-on Authentication
3) Set "Single Sign on Type" to disabled > Save
4) Log into USS. This time you won't see the popup window asking for credentials.