Issue
If a project has been version controlled using Microsoft Sharepoint, it would prompt for user authentication on new launch of same project.
Reason
You can use the select - Use Windows Authentication
As per design, RoboHelp remembers the credential in same session, when you re-launch the project in a new instance, the username from the last session is retrieved.
Additonal Information
User can select - Use Windows Authentication, which would allow the authentication to be validated using user's Windows login credentials.
Bug#3187117, has addresses the problem by saving the Username separately for SharePoint version control and publishing scenario.